Watson McDaniel Steam Design Guide

Steam Design Guide

A Handbook for the Proper Design of Reliable Steam and Fluid Systems

Made in the USA Since 1878

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Manufacturing High-Quality Steam & Fluid Specialty Products for Industry Made in the USA Since 1878

Steam Traps Condensate Pumps Pressure Regulators Temperature Regulators Control Valves Relief Valves Liquid Drainers Check Valves

For over 145 years, Watson McDaniel has been manufacturing a wide range of steam spe- cialty and fluid products for the industrial marketplace. These time-tested products have made the operation of steam, compressed air, heat transfer and fluid systems substantially more effec- tive and efficient. In 1995, Watson McDaniel received its ISO 9001 Quality Certification as industry rec- ognition of our continued commitment to world class manufacturing, assembly and quality con- trol procedures. This level of quality certification assures our customers unequaled dependability of our products. Our manufacturing facilities, with over fifty computer numerical controlled (CNC) machining centers, is considered the most modern in the industry. Watson McDaniel serves the global marketplace with a network of Manufacturers, Repre- sentatives, Distributors, Manufacturing Plants and Sales Offices located throughout the world. In 1997, a Manufacturing Plant and Sales Office was opened in Shanghai, China to fulfill the growing demands of steam specialty products in the Far East. The success of this operation has allowed us to quickly deliver products with competitive prices to our customers throughout this region. The structure of our operation affords us the ability to give highly personalized attention to each and every customer. We continually strive to provide ultimate customer service and pro- duct reliability while responding immediately to our customers' requests and detailed needs. Watson McDaniel welcomes the opportunity to work with your company so that we may help to make all of your steam system and fluid applications the best that they can be.

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Table of Contents Industrial Steam Book

Steam Traps

4—19

STEAM and CONDENSATE TUTORIAL

20—36

STEAM TRAPS

Steam Trap Types

20-26

Drip Applications

27-30

Tracing Applications

Process Applications

32-36

37—43

PUMP TRAPS

Overview

37-40

Stall Calculations

41-43

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Engineering Data

Table of Contents

Page No.

Formulas, Conversions & Guidelines

46 47

• Equivalents & Conversion Factors • Capacity Formulas for Steam Loads • Formulas for Control Valve Sizing

48-49

Steam Properties & Flow Characteristics • Properties of Saturated Steam

50 51 52 53 54 55 55 56

• Draining Condensate from Steam Mains or Steam Supply Lines

• Steam Capacity Tables

• Steam Flow thru Various Orifice Diameters

• Sizing Steam Pipes

• Sizing of Condensate Return Line, Vent Line & Flash Tank

• Percent Flash Steam Table

• Pressure Drop in Schedule 40 Pipe

III

Fluid Flow in Piping

• Flow of Water thru Schedule 40 Steel Pipe – Flow Rates, Velocities & Pressure Drops

Pipe, Fitting & Flange Specifications

58-60

• Pipe Data Table (for 1/8” thru 30” sizes)

• Maximum Allowable Working Pressures for Seamless Carbon Steel Pipe

62-63

• Flange Standards – Dimensional Data • Fitting Standards & Specifications

65-67

• Standard Class Pressure-Temperature Ratings

Heat Exchanger Formulas & Example • Formulas for Heat Exchanger System using a Modulating Control Valve

68-69 70-79

• Heat Exchanger Example: Heating Water with Steam using a Modulating Control Valve

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Hook-Up Drawings

Table of Contents

Page No.

Steam Trap Applications • Introduction to Steam Traps

82 84 86 88

• Drip Leg Design

• Process Trap Guidelines – Gravity Drainage • Process Trap Guidelines – Syphon Drainage

VII

Regulating Valve Applications • General Regulator Application & Installation Notes • Pressure Reducing Station using Spring-Loaded Pilot

90 92 94 96 98

• Pressure Reducing Station using Spring-Loaded Pilot & Trip-Stop Valve • Pressure Reducing Station using Air-Loaded Pilot for Remote Installations • Pressure Reducing Station – Two-Stage (Series) for High-Pressure Turndown

100 102 104 106 108 110 114 116 118 120 122 124 126 128 130 132 134 136 138

• Pressure Reducing Station – Parallel for High-Flow Turndown

• Pressure Reducing Station – Parallel for High-Flow Turndown to Deaerator • Pressure Reducing Station – Two-Stage Parallel for High-Pressure & High-Flow Turndown • Temperature Control of a Batch Process with Electrical Time Sequence Programmer (Solenoid Pilot) • Temperature Control of a Semi-Instantaneous Heater using a Self-Contained Temperature Regulating Valve • Temperature Control of a Semi-Instantaneous Heater using a Pilot-Operated Temperature Regulating Valve Control Valve Applications • Temperature Control of a Semi-Instantaneous Heater using an Electrically-Actuated Temperature Control Valve • Temperature Control of a Semi-Instantaneous Heater using a Pneumatically-Actuated Temperature Control Valve

VIII

• Temperature Control of an Air Heating Coil using a Temperature Control Valve • Pressure Reducing Station using a Pneumatically-Actuated Control Valve

• Boiler Feed using a Water Control Valve with Cavitation Control

• Flow Mixing using a 3-Way Control Valve • Flow Diverting using a 3-Way Control Valve

Pressure Motive Pump (PMP) & Pump-Trap Applications • Condensate Recovery using a Pressure Motive Pump • Drainage of a Single Source of Condensate using Pump-Trap • Drainage of Condensate from Below Grade using Pump-Trap

• Drainage of Condensate from Heat Exchanger Positioned Close to the Ground

• Flash Steam Recovery

• Removal of Water or Condensate from a Pit

140 — 148

PRODUCT CROSS REFERENCE

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Steam & Condensate Introduction

Water is heated to its boiling point, producing steam

What is Steam? Steam is simply the gas that is formed when water is heated to its boiling temperature at a given pressure. A tea kettle is the most common example of producing steam by heating water to its boiling temperature (212˚F). In this case, the steam does not develop any pressure and is released into the atmosphere. A boiler will generate steam under pressure by heating a large quantity of water in a contained system. This pressurized steam will travel throughout the pipes in the system to where it is needed. In addition to being created from water, which is readily available and relatively inexpensive, steam has many other advantages that make it easy and efficient to work with.

WATER

STEAM

Steam condenses and turns back to water

What makes steam desirable to use for heating? Another benefit of using steam is that steam temperature is directly related to the pressure of the system. Therefore, by increasing or reducing pressure, it is easy to increase or reduce the temperature.

The Steam & Condensate Loop

Steam transfers its heat into the room, and then it condenses back into water and collects at the bottom of the radiator. This water is referred to as condensate. The condensate must be continually removed or the radiator will fill up with water. This is the purpose of a steam trap.

The radiators are therefore heated by the steam and give off heat into the room

Condensate drains from the radiator

Steam travels up the pipe and fills the radiators with steam

Steam Trap

Steam

Boiler

A Steam Trap allows condensate to be discharged from the radiator and returned back to the boiler. It will not allow steam to pass. The steam trap closes when steam is present (trapping the steam) and opens when condensate is present (discharging the condensate).

Pump

Water

Steam Trap

Water is heated under pressure inside of the boiler and turned into steam

A pump may be used to return condensate to the boiler

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Steam & Condensate Introduction

Pressure / Temperature Relationship of Steam Steam is created when water is heated to its boiling temperature until enough heat energy is absorbed to transform the water from a liquid to a gas. The temperature at which water boils is 212˚F; however, this is the boiling point of water at 0 psig, or atmospheric pressure. A unique property of steam is that there is a direct relationship between the pressure at which it is generated and the temperature at which it boils.

The boiling temperature increases as steam pressure increases. If steam is generated at a pressure higher than 0 psig, the temperature at which the water boils will be higher than 212˚F. An abbreviated version of the Saturated Steam Table is included to show the exact boiling temperature at various steam pressures. (The complete steam table is available in Engineering Section.)

Temperature (˚F )

Steam Pressure (psig)

Steam

Boiling Water

0 psi = 212˚ F 1 psi = 215˚ F 4 psi = 224˚ F 10 psi = 239˚ F 50 psi = 298˚ F 100 psi = 338˚ F 150 psi = 366˚ F 200 psi = 388˚ F 300 psi = 421˚ F

212˚ F 215˚ F 224˚ F 239˚ F 298˚ F 338˚ F 366˚ F 388˚ F 421˚ F

Where else is steam used? Hospitals and pharmaceutical manufacturers may use steam for the sterilization of medical instruments and production of medicines, while the petrochemical industry may use steam for processing gasoline from crude oil. Steam is essential in large scale food processing & manufacturing applications. Large cities, such as New York, have centralized steam systems for heating large apartment complexes. Heating Properties: The energy absorbed by water at its boiling point to transform it from a liquid to a gas is known as Latent Heat . This Latent Heat is then released by the steam when used for heating. Steam is very efficient in transferring heat to other processes. Steam, being a gas, allows it to surround any surface it needs to transfer its heat energy into. When steam transfers its heat, it condenses back into water, which will be drained away and sent back to the boiler in order to be used again (referred to as Condensate Recovery). Steam Supplies Heat at a Constant Temperature Steam does not reduce its temperature when it releases its heat; it just simply changes from a gas back into water at the same temperature. For example, steam at 50 psig (is at 298˚F; refer to steam chart above) will condense back to water at 298˚F when it releases its heat energy. In contrast to steam, water reduces in temperature when it gives up its heat. What is saturated steam? Steam that is generated under pressure inside the boiler, while in the presence of boiling water, is referred to as Saturated Steam . If additional heat is later added to the saturated steam to increase its temperature, it is then referred to as Superheated Steam. Superheated steam is used in power generation and saturated steam is used for heating. When saturated steam releases its energy, it condenses back to water. This hot water at or near boiling tem- perature is referred to

Steam Turbines in Power Plants

Steam exhaust from power plants

Steam used in cities for heat

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Steam & Condensate Introduction

Steam Traps

Typical equipment used for process heating in steam systems A steam jacketed kettle contains a liquid to be heated surrounded by an isolated jacket containing the steam (steam does not contact the fluid). They are typically found in commercial food processing facilities. The Shell & Tube Heat Exchanger is used for continuous processes where a liquid to be heated (such as water), continually flows through the tubes surrounded by the steam.

Steam Jacketed Kettle

Shell & Tube Heat Exchanger (Single tube shown for clarity)

Pressurized Steam enters

Steam is contained in a “jacket” which surrounds the Kettle and heats the contents indirectly through the metal wall

Shell contains the steam which surrounds the tube section in order to heat the liquid flowing thru it

Pressurized Steam enters

Liquid to be heated

Hot Water Exits

Single Tube shown for clarity

Cold liquid enters the tube section & the heated liquid exits

Cold Water Enters

Condensate discharges from below

Typical pieces of equipment used to control, protect and optimize steam systems Now that a basic understanding of steam has been provided, let’s introduce some components of the system and their general purposes:

Steam Traps

Since steam is created from water, it will condense back to water after releasing its energy during heating. This water, or condensate, must be removed to not only ensure proper heat transfer, but system safety as well. Removing condensate without the loss of live steam is the primary function of Steam Traps. Steam traps also discharge air that is present in the system prior to system start-up.

Pressure Regulators & Control Valves

Steam is generated at the boiler at pressures sufficient to ensure travel throughout the entire piping system. Pressure Regulating Valves and Control Valves may be used for temperature control or to reduce the steam pressure generated at the boiler down to more usable levels.

Condensate Return Pumps

When condensate does not have sufficient pressure to return to the boiler on its own, mechanical or electric pumps are required to pump the condensate back to the boiler.

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Steam & Condensate Introduction

How does steam flow in a system? Steam coming from the boiler is distributed throughout the system by pipes referred to as steam mains or steam supply lines. Since steam is generated under pressure at the boiler, it will travel on its own through the system. Steam may travel in pipes at velocities exceeding 90 mph ; for this reason, care should always be taken to open and close valves slowly. What is condensate and why must it be removed from a system? When steam releases its heat energy, it condenses from a gas back to a liquid. This “condensed” steam is referred to as condensate ... which is nothing more than extremely hot water. As previously discussed, steam at 50 PSIG condenses back into water at 298˚F. Steam Traps were specifically designed for the removal of unwanted Condensate and Air . Condensate will form in steam pipelines due to radiation losses through the pipe walls. Drip Traps remove condensate from steam pipelines. However, the bulk of the condensate formed in the system occurs in the heat exchangers and other processes, and must be removed or the system would fill with water and impede the heat transfer process. In contrast to drip traps, Process Traps remove condensate from the actual process application (such as a heat exchanger).

System showing use of Steam for Heating in two different Process Applications: Steam Jacketed Kettle & Tank with Steam Coil (Vat Process)

50 psig Inlet Pressure

Roof

Steam travels through piping

Process 2

Process 1

Vat Process

Steam Jacketed Kettle

Drip Trap TD600S

Steam turns back into condensate in the Jacketed Kettle just as it did in the radiator illustration

Condensate

Process Steam Trap

Condensate

Vented Receiver

Boiler Feed Tank

Boiler Feed Pump

Pump

Condensate

Condensate Return Pump

Note the process steam traps draining condensate from the Steam Jacketed Kettle and the Vat Process, discharging into a condensate return line. Condensate is then drained into a vented receiver which is used to release flash steam from the hot condensate in order to neutralize the pressure in the condensate return line. Also note the drip traps used for draining condensate from the steam supply lines. Other components, such as control valves and pressure regulating valves that would be required to control steam pressures and product temperatures, have not been included for simplification purposes.

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Steam & Condensate Introduction

WHY ARE STEAM TRAPS REQUIRED? The purpose of the steam trap is to allow Condensate (water that is formed from the condensed steam) and air, to be discharged from the steam system while preventing the loss of live steam. The steam trap is a special type of valve which opens when condensate and air are present and closes when steam tries to pass. CONDENSATE: (condensed steam or water): Any time steam releases its heat energy (latent heat), the steam condenses back to water. This water is therefore referred to as condensate. This transformation of steam back to liquid condensate will occur in a radiator heating a room, in a heat exchanger making hot water, in a pipe transferring the steam over long distances, or in any process that uses steam. If this condensate is not continuously removed, the radiators, heat exchangers and piping will fill with condensate (water). The removal of condensate from the steam system, while preventing the loss of live steam, is therefore the primary function of the steam trap. AIR: Before the steam is turned on and the system is cold, air will exist in all the steam pipes and process equipment, such as radiators and heat exchangers. This air must be bled from the entire system to allow the steam to enter and reach its intended designated process. The air is actually pushed thru the system by the incoming steam and automatically bled thru the process traps at the end of the steam lines or special air vents at the high points in the system. This bleeding of air from the system allows the steam to enter. GENERAL APPLICATION CATEGORIES for STEAM TRAPS: DRIP APPLICATIONS: Drip applications refer to removing the condensate that forms in the steam main and steam supply lines as opposed to condensate that forms at the actual process (heat exchanger, jacketed kettle, radiator, etc.). When steam loses its heat energy due to radiation losses through the pipe walls, condensate forms in the pipes. This condensate needs to be continuously removed, and it is therefore common to have steam traps placed 150–300 feet apart throughout the piping system. Traps used for this application are referred to as drip traps and have small condensate capacities as opposed to process traps. Drip traps are not normally relied upon to discharge the air from the system. Air removal is performed by the process traps and air vents located throughout the system. The most common trap choices for drip applications are the Thermodynamic style for line pressures over 30 PSIG, and Float & Thermostatic style for line pressures up to 30 PSIG. Inverted Bucket (IB) style traps are also commonly used for drip trap applications. The orifice of the IB is mounted at the top of the trap which makes them less susceptible to failure from dirt and pipe scale when compared to other trap types. PROCESS APPLICATIONS: Process applications refer to removing condensate and air where the actual process using the steam is taking place. This process could be a heat exchanger making hot water, or a radiator heating a room, or anything else that requires the use of steam. Traps used for process applications require larger condensate handling capability in contrast to steam traps that are used for drip applications. Traps used in Process applications also need to be able to discharge large amounts of air present in the system at start-up. The most common trap choice for process applications are Float & Thermostatic traps since they do an excellent job of discharging condensate and air. Thermostatic traps make a good choice for process applications since they also do an excellent job of discharging air and condensate. In contrast, the lack of air venting capability of the Thermodynamic and Inverted Bucket traps, make these trap types a less desirable choice for most process applications.

Common Types of Steam Traps

Shown below are some of the most common types of steam traps; Float and Thermostatic, Thermodynamic, Thermostatic, as well as a Thermostatic Air vent. Other common steam trap types are the Inverted Bucket and the Bi-Metal. In the following diagrams, other system components such as control valves and regulating valves are often required to control steam pressure and process temperatures. (Some piping components may not be included in the diagrams for simplification purposes.)

Thermostatic Trap Thermodynamic Trap Float &Thermostatic Trap Thermostatic Air Vent

Contains a float-operated valve to discharge condensate, and a thermostatic air vent which discharges air, but will close when steam is present.

Contains a thermostatic element which allows air and condensate to be discharged, but closes when steam is present.

Contains a disc and seat arrangement which allows condensate to be discharged, but will close when steam tries to pass through.

Air Vents are used in steam systems for the removal of air and other non- condensable gases. They are placed at the end of steam mains and directly on process equipment.

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Diagram of a Steam System

DRIP APPLICATION:

AIR VENT: Placed at the end of steam mains and other high points in order to remove air from the system.

Drip applications refer to removing the condensate that forms in the steam pipes as opposed to condensate that forms at the actual process. It is appropriate to have steam traps (drip traps) placed 150 to 300 feet apart.

Air Vent OPEN Discharging Air on start-up

Trap CLOSED

Trap OPEN

Air

steam

CLOSED Trapping steam in system

OPEN Discharging Condensate

Roof

Air Vent Discharges air from the system (placed at end of steam main)

AV2000

Drip Application: Removes condensate formed in the steam main

Process #2 Vat Process

Process #1

(placed every 200 feet)

Steam Jacketed Kettle

Drip Trap TD600S

Process Application: Removes condensate formed by the process

F&T Trap

Process Trap

Drip Application: Removes condensate formed in the steam supply line

Process Trap

Drip Trap TD600

Condensate pumped back to the boiler room to be reused

PROCESS APPLICATION:

Steam Jacketed Kettle

Air Vent OPEN Discharging Air on start-up

Trap CLOSED Air Vent CLOSED Trapping steam in system

Air Vent CLOSED when steam is present

AIR

STEAM

condensate

Trap OPEN Discharging Condensate

Air

Process Applications refer to removing condensate and air from the actual process where steam is being used. This process could be a heat exchanger making hot water, or a radiator heating a room, or anything else that requires the use of steam. Traps used for process applications require larger condensate handling capability than steam traps used for drip applications and also need to be able to discharge large amounts of air.

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Steam & Condensate Introduction

Steam Traps

Operation of a Steam System

How does condensate flow through steam traps? Steam Pressure pushes the condensate through the trap .

Every steam trap has an Inlet Pressure (Steam Supply Pressure) and an Outlet Pressure . The difference between inlet & outlet pressure is referred to as the Differential Pressure . When the Inlet Steam Pressure is higher than the Outlet Pressure (Positive Differential Pressure), the steam will “PUSH” the condensate through the steam trap.

Outlet Pressure

Inlet Pressure 50

Differential Pressure is an important factor for sizing steam traps as well as other components, such as regulators and control valves. The higher the Inlet Pressure in relation to the Outlet Pressure, the more condensate the trap can remove from the steam system. The trap capacity is therefore a function of the differential pressure across the trap.

Inlet Pressure (Steam)

Differential Pressure

Outlet Pressure (Condensate)

–

Steam pressure pushes condensate through the trap

50 psig

– 0.0 psig

= 50 psi

50 psig Inlet Pressure

Roof

Air Vent Open Discharges air during start-up

0 psig

Outlet Pressure

Process #2

Process #1

Vat Process

Steam Jacketed Kettle

Drip Trap TD600S

0 psig

Process Trap

Steam

Drip Trap TD600

Vented Receiver

Condensate Return Line Condensate flows by gravity into the flash vessel .

Boiler

Pump

Condensate Return Pump A mechanical or electric Pump is used to return the condensate.

Vented Receiver A Vented Receiver maintains a pressure of 0 psig inside the condensate return lines by venting the flash steam generated by

the hot condensate to the atmosphere.

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Steam & Condensate Introduction

Operation of a Steam System

DRIP APPLICATION using a Thermodynamic Trap: Removing condensate from steam mains & steam supply lines Drip applications refer to removing the condensate that forms in the steam pipes (due to heat losses) as opposed to condensate that forms at the actual process. It is appropriate to have “Drip traps” placed 150 to 300 feet apart in the steam pipe line, and at any abrupt changes in direction or elevation. Air discharges through the separate air vent located at the end of the steam line.

Thermostatic Air Vent OPEN when air is present and CLOSED when steam tries to escape.

The Steam Trap discharges the Condensate & the Air Vent purges the air.

Incoming steam pushes air through the OPEN air vent.

AIR

STEAM

Outlet Pressure

Inlet Pressure

Drip Leg

Steam Pressure pushes condensate through the steam trap

Inlet Pressure (Steam)

Differential Pressure

Outlet Pressure (Condensate)

–

50 psig

– 0.0 psig

= 50 psi

Thermodynamic Steam Trap (TD600S) used in Drip Application

Condensate Discharge

PROCESS APPLICATION using a Float & Thermostatic (F&T) Trap : Removing condensate and air from a steam jacketed kettle

Operation – Condensate discharging from system Steam now fills the jacket at full operating pressure, heating the contents of kettle. Steam is condensing and the steam pressure in the kettle is being relied upon to push the condensate through the steam trap and into the condensate return line.

Start-Up – Air discharging from system Air that entered the system during system shut-down must be purged so that steam may enter. Float & Thermostatic steam traps contain a separate thermostatic air vent for discharging air during system start-up. Note: Additional air vents may be installed on the process or other high points in the system.

Inlet Pressure

Steam pushes air through the Air Vent in the Steam Trap and also through the auxiliary air vent.

Air

Steam is now at Full Pressure and all Air is discharged from the system

Air Discharging Air Vent placed at high point in system. Air vent is Open when air is present.

Air Vent CLOSED

Steam Jacketed Kettle

Inlet Pressure

Air Discharging Steam Pressure pushes air through the Thermostatic Air Vent in the steam trap

Steam Pressure pushes condensate through the steam trap

Air

Float & Thermostatic (F&T) Steam Trap contains separate thermostatic air vent

Float & Thermostatic Steam Trap

Air

Outlet Pressure

Discharging Condensate

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Steam & Condensate Introduction

Typical Ways Steam Traps are Installed ... and how this affects the differential pressure.

Depending on the installation of the steam trap, the pressure at the outlet of the trap can vary significantly. It is important to understand the trap Outlet Pressure as this will affect the differential pressure used for sizing and selecting the appropriate steam trap. Furthermore, there could be instances where steam supply pressure to the inlet of the trap is insufficient to "push" the condensate into the return line. The following diagrams show: 1) discharging condensate to atmosphere, 2) discharging condensate into gravity return line, and 3) discharging condensate into an elevated and/or pressurized return line.

1) Discharging Condensate to Atmosphere:

Discharging condensate to atmosphere is often done in larger facilities when it may not be cost-effective or practical to install long lengths of condensate return lines back to the boiler. The Pressure in the steam main. In our case, 50psig Outlet Pressure: Since we are discharging steam trap to atmosphere, 0.0psig Inlet Pressure: Steam Pressure “pushes” the condensate through the steam trap allowing it to discharge out of the system (0 PSIG)

50 psig

Inlet Pressure

50 psig

0 psig Outlet Pressure

Inlet Pressure (Steam)

Outlet Pressure (Condensate)

–

Differential Pressure

Condensate Discharge to Atmosphere

TD600S Thermodynamic Steam Trap

50 psig

– 0.0psig

= 50 psi

2) Discharging Condensate to Gravity Return Line (Connected to Vented Receiver):

50 psig

It is always preferable to drain condensate in the direction of gravity to a condensate return line which leads into a vented receiver for condensate collection. In most situations the vented receiver vents to atmosphere, and is therefore at a pressure of 0.0 psig. Steam Pressure “pushes” the condensate through the steam trap allowing it to discharge into gravity return line (0 PSIG)

Inlet Pressure

Venting Flash Steam

50 psig

Outlet Pressure

0 psig

The Pressure in the steam main. In our case, 50 psig

Inlet Pressure:

Inlet Pressure

50 psig

Since the steam trap is being discharged to a properly sized condensate return line that leads to a vented receiver, we assume 0.0psig

Outlet Pressure:

Condensate flows by gravity to the vented receiver

0 psig

0 psig Outlet Pressure

Vented Receiver

Inlet Pressure (Steam)

Outlet Pressure (Condensate)

–

Differential Pressure

50 psig

– 0.0psig

= 50 psi

Pump

Condensate returns back to boiler

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Steam & Condensate Introduction

3) Discharging Condensate into an Elevated and/or Pressurized Return Line:

Total Back Pressure (Outlet Pressure) is the Sum of Condensate Return Line Pressure + Equivalent Lift Height Pressure Discharging condensate upward against gravity is the least desirable scenario; however, in certain instances, it may be the only solution possible. Since condensate must be “lifted” to an elevation, it adds additional back-pressure to the discharge (outlet) side of the trap. For this example the condensate return line pressure is 5 psig. We first need to calculate Lift Height Pressure : Height x 0.433 Lift Height Pressure = Steam Pressure “pushes” the condensate up through an Elevated return line (15 PSIG)

Condensate Return Lines are designed to drain by gravity at 0 psig; however, they often contain unintentional pressure from being undersized or from the discharge of failed steam traps.

5 psig

50 psig

23.1 ft. Equates to

23.1 Ft. Lift Height

23.1 Ft. X 0.433

10 psig Pressure

= 10 psig

Pressure in the steam main. = 50 psig

Inlet Pressure:

Inlet

Since we are discharging steam trap to a pressurized and elevated condensate return line, we need to add condensate

Outlet Pressure 5 + 10 = 15 psig

50 psig 15 psig

Outlet Pressure:

Total Back Pressure

return line pressure (5 psig) to lift height pressure (10 psig) = 15 psig

TD600 Thermodynamic Steam Trap

Inlet Pressure (Steam)

Outlet Pressure (Condensate)

–

Differential Pressure

50 psig

– 15psig

= 35 psi

Calculating Lift Pressure A column of condensate in vertical piping results in additional pressure at the outlet of the steam trap. By knowing the height of the condensate return line, the pressure of this column can be easily calculated as follows: Lift pressure (psig) = Lift height (ft) x 0.433 (psig/ft)

23.1 Ft. X 0.433 = 10 psig

11.55 Ft. X 0.433 = 5 psig

Weight of the water column creates pressure

A column of water exerts a downward pressure of 1 psi for every 2.31 ft. of height

23.1 ft. water column

11.55 ft. water column

2.31 Ft. X 0.433 = 1 psig

5 psig

10 psig

1 psig

2.31 ft. water column

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Steam & Condensate Introduction

Steam Trap Installed after a Control Valve ... which can cause wide variations of trap inlet pressures and condensate loads.

The flow rate and the steam pressure in the jacketed kettle is determined by the temperature control valve. When the process fluid in the jacketed kettle reaches the desired set temperature, the control valve reduces the flow of steam which, in turn, reduces steam pressure. The Steam pressure can drop down to 0 psig or below (to sub-atmospheric pressures) to maintain just the correct amount of steam flow to keep the kettle at the exact set temperature. With the varying amount of steam that is sent to the process, the amount of condensate that is generated also varies. If the steam demand is high for a given period, more condensate is generated after the steam is used. When there is a low steam demand, less condensate is generated. The appropriate steam trap selected for process applications must be able to adjust to varying condensate loads without oversizing, and have the capability to remove air from the system.

The Control Valve regulates the amount of Steam delivered to the process equipment

Temperature Controller

50 psig

Flash Steam

Temp Sensor

30 psig

Steam Pressure pushes condensate through the steam trap

0 psig

2.3 FT.

Flash Steam

0 psig

F&T Trap

Vented Receiver A Vented Receiver maintains a pressure of 0 psig inside the condensate return lines by venting the flash

Hot Condensate

Inlet Pressure (Steam)

Outlet Pressure (Condensate)

–

Differential Pressure

steam generated by the hot condensate to the atmosphere

30 psig

–

0psig

= 30 psi

Pump

Why the Steam Trap needs to be placed a minimum distance below Jacketed Kettle

0 psig

When set temperature of the process fluid is reached, the steam pressure inside the jacketed kettle may reduce to 0 PSIG or even go into Vacuum. To promote condensate drainage, the steam trap is placed a certain distance below the process equipment. 2.3 ft. will provide 1 psig of condensate head pressure. As long as the trap discharges into a gravity return line (at 0 psig), there will be 1 psi differential pressure and condensate may freely drain. Pressure = Column Height x 0.433 psi ft 1 psig = 2.31 Ft. x 0.433

Inlet 1 psig

1 psig

2.31 ft. water column

2.3 ft.

2.31 Ft. X 0.433 = 1 psig

www.watsonmcdaniel.com •• Pottstown PA • USA • Tel: 610-495-5131

Steam & Condensate Introduction Typical Process Equipment Which Use Steam for Heating

Inverted Bucket IInverrtted Buckett

Batch Processes: Steam Jacketed Kettle

Steam jacketed kettles are used for batch processing and are typically found in commercial food processing facilities. A steam jacketed kettle contains a liquid to be heated surrounded by an isolated jacket containing the steam (steam does not contact the fluid). Steam enters the kettle and its heat is then transferred to the liquid through the jacket wall and the condensate is discharged out the bottom. Steam Pressure to the kettle is controlled by the Steam Supply (Control) Valve. The steam trap is placed a minimum distance below the kettle to promote condensate drainage when low pressure or partial vacuum exists in the jacket of the kettle (14” is equivalent to 1/2 psi of head pressure).

Pressurized Steam enters Jacket of Kettle

Steam is contained in a “Jacket” which surrounds the Kettle and heats the contents.

Liquid to be heated

Condensate discharges from below

Steam Trap (not shown) placed a minimum distance of 14” below kettle to promote drainage of condensate under vacuum.

Continuous Process: Shell & Tube Heat Exchangers

Shell & Tube Heat Exchangers are used for continuous processes such as heating a continuous flow of water or other liquid. The Shell & Tube heat exchanger contains multiple tubes inside to optimize heat transfer to the process. In the majority of applications, the process liquid goes through the inside of the tubes and the steam surrounds the outside of the tubes and is contained within the shell area. The condensate that is formed from the condensed steam is discharged out of the bottom through a steam trap. Steam Pressure to the heat exchanger is con- trolled by the Steam Supply (Control) Valve. The steam trap is placed a minimum distance below the heat exchanger to promote condensate drainage when low pressure or partial vacuum exists in the shell of the heat exchanger (14” is equivalent to 1/2 psi of head pressure).

Shell contains the steam which surrounds the tube section in order to heat the liquid flowing thru it

Pressurized Steam enters Shell of Heat Exchanger

Single Tube shown for clarity

Cold liquid enters the tube section & the heated liquid exits

Tube Bundle of a Shell & Tube Heat Exchanger

Condensate discharges from below

Steam Trap (not shown) placed a minimum distance of 14” below Heat Exchanger to promote drainage of condensate under vacuum.

Shell & Tube Heat Exchanger

Tel: 610-495-5131 • Pottstown PA • USA •• www.watsonmcdaniel.com

Steam & Condensate Introduction

Batch Process Application: Jacketed Kettle ... from Start-Up to Reaching Temperature Set Point Let’s take a detailed look at a batch process application using a control valve to heat the contents of a Jacketed Kettle to a specific temperature. Steam will enter the jacket to indirectly heat the kettle contents through a metal wall. Temperature Controller Steam Control Valve Temp Senso r Vacuum Breaker

Air Vent

The condensate load and pressure drop across the steam trap varies because the control valve will open and close in response to the temperature of the contents inside of the kettle. As the valve opens and closes, the steam pressure and steam flow in the jacket will vary, affecting the differential pressure across the steam trap and condensate load requirements. A Float & Thermostatic steam trap is the primary choice for the majority of process applications because of its ability to quickly adjust to changing condensate loads, as well as having the capability to discharge air from the system.

Steam

Steam Jacketed Kettle

Steam Pressure in Jacket of Kettle

F&T TRAP

DIAGRAM 1: Start-Up (Air Vents Open)

DIAGRAM 2: Steam Enters (Trap Fully Open; Air Vents Closed)

On start-up, jacket is filled with air which must first be dis- charged by the Air Vents to allow steam to enter for heating. Float & Thermostatic steam traps contain a separate thermosta- tic vent, and can discharge large volumes of air present during system startup. Additional air vents may be installed on the kettle. The faster air is expelled, the faster steam can enter and heating can begin.

Once the air has been discharged, steam can fill the jacket. Since the kettle is cool, the control valve will open to allow as much steam as possible to fill the jacket and begin heating the contents in the kettle. The steam trap must adjust to the high condensate load as the steam is entering and building pressure.

Air Vent OPEN Discharges air during start-up

Air Vent CLOSED Steam Temperature Closes Air Vent

Air Vent CLOSED

Air Vent OPEN

Steam

Air

Presence of air prevents heating

Heating begins

Air Vent CLOSED When steam is present

Air Vent OPEN Discharges air during start-up

Air

Steam Pressure in Jacket of Kettle (low pressure)

Steam Pressure in Jacket of Kettle (high pressure)

Air

Control Valve FULLY OPEN

Air

Trap Fully OPEN Discharging Condensate

Air Discharging from Process on Start-Up

Temperature of Steam causes Air Vents to Close

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Steam & Condensate Introduction

DIAGRAM 3: Nearing Set Temperature (Trap Partially Open) As the temperature of the kettle contents

nears set point, less steam will be required and the control valve will modulate toward a partially open position. As this happens, steam pressure decreases in the jacket and therefore the pressure differential across the steam trap will likewise decrease. The steam trap will then adjust to the lower condensate flow generated.

Air Vent CLOSED

Steam Pressure in Jacket of Kettle (low pressure)

Control Valve PARTIALLY OPEN

Trap Partially OPEN

Process Liquid is nearing Set Temperature

DIAGRAM 4: Temperature Set Point Achieved (Steam Flow Reduced; Since Only Required to Maintain Temperature) Once the set temperature is achieved, a significantly less amount of steam is required to maintain the temperature of the product inside the jacketed kettle. The steam supply valve will modulate to a near shut-off condition, dropping the pressure, and the kettle may be operating in vacuum. This action will impede the discharge of condensate as the pressure in the jacket will be less than atmospheric. Therefore, a vacuum breaker is required to allow air to enter the jacket and equalize the pressure. This then allows drainage of condensate through the steam trap by gravity. If the vertical discharge leg from the jacket is 2.3 ft., this will provide 1 psi head pressure to assist with condensate drainage.

System without Vacuum Breaker

System with Vacuum Breaker

Vacuum Breaker OPEN Vacuum breaker equalizes the pressure in the jacket and allows head pressure to "push" condensate through the steam trap

Condensate does not freely drain because of negative pressure differential (i.e. atmospheric pressure at 0 psig is higher than jacket pressure at -5 psig )

Condensate Backs Up into Jacket of Kettle

-5 psig

0 psig

Steam Pressure in Jacket of Kettle (vacuum)

2.31 Ft.

Steam Pressure in Jacket of Kettle (atmospheric) (0 psig)

Control Valve PARTIALLY OPEN

1 psig

Control Valve PARTIALLY OPEN

Condensate

Condensate does not drain freely

0 psig

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Steam & Condensate Introduction

Continuous Process Application: Shell & Tube Heat Exchanger

Same as inlet pressure to steam trap

Steam pressure to Heat Exchanger

Let’s take a detailed look at a continuous process application using a control valve on a Heat Exchanger to heat a variable flow rate of water to a constant temperature. Cold water enters the Heat exchanger and hot water is discharged at an elevated temperature. The condensate load and pressure drop (differential pressure) across the steam trap are not constant. Therefore, it is important to select a steam trap that can handle high condensate loads at very low pressure drops, without significantly oversizing the steam trap during normal operation. A temperature control valve will modulate between an open and closed position to deliver the proper amount of steam to a heat exchanger to maintain the outlet water at a desired temperature. During this process, the steam pressure in the heat exchanger will vary depending on the flow rate of heated water produced. The higher the flow rate of water – the higher the steam pressure in the heat exchanger will be. Conversely, when water flow is reduced, steam pressure is reduced.

Inlet pressure to Control Valve

Steam Control Valve

Temperature Controller

Air Vent

Vacuum Breaker

Steam

Temp Sensor

Heat Exchanger

hot water outlet

F&T TRAP

cold water inlet

DIAGRAM 1 : Start-Up (Air Vents Open)

DIAGRAM 2 : Steam Enters (Trap Fully Open; Air Vents Closed) Since the water temperature is cold, the control valve is fully open to allow as much steam as possible to fill the heat exchanger. The steam trap must adjust to the high condensate load as the steam is entering and building pressure. This steam pressure in the shell of the heat exchanger pushes the condensate through the steam trap and into the return line.

On start-up, heat exchanger is filled with air which must first be discharged by the Air Vents to allow steam to enter for heating. Float & Thermostatic steam traps contain a separate thermostatic vent, and can discharge large volumes of air present during system startup.Additional air vents may be installed on the heat exchanger. The faster air is expelled, the faster steam can enter and heating can begin.

Air Vent CLOSED Steam Temperature Closes Air Vent

High Pressure

Same as Inlet pressure to Steam Trap Low Pressure

Inlet pressure to Control Valve

Air Vent OPEN Releases air during start-up

Steam pressure to Heat Exchanger

Air Vent Closed

Air Vent Open

Air

STEAM

Trap CLOSED Air Vent OPEN Discharging Air on start-up

AIR

Air Vent CLOSED when steam is present

STEAM

AIR

condensate

Trap Fully OPEN Discharging Condensate

Air

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Steam & Condensate Introduction

DIAGRAM 4 : High Running Load

DIAGRAM 3 : Typical Running Load

When a high flow rate of heated water is required, the control valve will open accordingly to allow more steam (lbs/hr) and steam pressure (psi) to enter the heat exchanger. During times of high water usage, there will also be a significant increase in the condensate load (lbs/hr), as well as higher steam pressure in the shell of the heat exchanger. This high pressure steam will push the condensate through the steam trap .

The temperature control valve will automatically adjust the flow of steam (lbs/hr) to coincide with the flow rate of heated water (GPM). The higher the flow rate, the higher the steam pressure will be. The steam pressure in the shell of the heat exchanger is indirectly determined by the amount of water flowing through the heat exchanger. The steam (lbs/hr) turns into condensate (lbs/hr) and is discharged through the steam trap.

Inlet Steam Pressure varies directly with water flow rate

50 psig

30 psig

10 psig

STEAM

Typical flow rate of hot water

High flow rate of hot water

Condensate flow through trap matches the steam load

DIAGRAM 5 : Low Running Load

When the demand for hot water is low, the steam control valve will adjust accordingly, allowing just enough steam to heat the reduced flow of water. The pressure in the shell of the heat exchanger will go into vacuum, preventing discharge of condensate. Therefore, a vacuum breaker is used to allow air to enter the shell and equalize the pressure, allowing drainage of condensate through the steam trap by gravity.

System with Vacuum Breaker

System without Vacuum Breaker

Sub-atmospheric Pressure

0 psig

OPEN Vacuum Breaker

- 5 psig

Vacuum causes Condensate to Back Up into Heat Exchanger

1 psig Head Pressure

1 psig

Low flow rate of hot water

2.31 ft. water column

Condensate will not flow without a vacuum breaker

Condensate flows through trap

0 psig

2.31 Ft. X 0.433 = 1 psig

Tel: 610-495-5131 • Pottstown PA • USA •• www.watsonmcdaniel.com

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