Machinery's Handbook, 31st Edition
PRESSURE REGULATION 2757 regulator is preferred for use with hazardous, explosive, or expensive gases since a reliev- ing regulator will vent excess pressure to atmosphere. The temperature range that the regulator will see is also a factor. Extreme operating tem peratures may affect flow capacity and/or the spring rate. Flow Measurement.— There are many ways to measure the flow of liquid or gas. Mea surement can be in terms of velocity, volume flow rate, or mass flow rate. When selecting a flow meter, one must consider the viscosity and chemical compatibility of the fluid being measured, the desired output units, the range of flow values being measured, the desired accuracy, and the maximum pressure encountered by the unit. The ratio of the maximum to the minimum flow rate a meter can measure within the expected accuracy is called “turndown.” When designing a pipe system, it is often beneficial to run a bypass system of pipes with valves to allow service of a flow meter without interrupting system flow. Flow meters should be placed in straight sections of pipe for best measurement results. The length of straight pipe before and after each meter will depend on its type, the pipe diameter, and the type of nearest fitting or bend. Pitot Tubes: Some pitot tubes are available to measure liquids and others are made for gases. These devices measure fluid flow velocity directly as a function of the height of a stationary column of fluid pushed out of the flow stream into a narrow tube. Most of the fluid flows past the device, but a small amount is captured in the static tube. Pitot tubes can be permanently installed in a pipe or duct, or used as a portable device for insertion as needed. The location of the capture tip of the pitot tube is critical, since flow velocity in a pipe varies across its cross section. Pitot-static tubes have ports perpendicular to the fluid flow direction to measure the static pressure of the fluid. Velocity is then calculated using the following equation: v 2 p t p s – ( ) ρ = ------------ where v is fluid velocity at the collection point; p s is static pressure; and p t is the stagnation pressure measured as a function of displacement of the fluid in the tube. Cup Anemometer: Air velocity is often measured with one of these devices. They consist of several cups arranged about a shaft that is free to spin. As air flows perpendicularly to the cups, the device rotates at a speed dependent on the air flow velocity. Hot Wire Anemometer: These devices employ a thin wire inserted into a gas stream. The wire is heated by current passing through it, and the gas flowing by tends to dissipate this heat. Some hot wire anemometers apply a fixed current, and thus the change in resistance of the wire (related to temperature change) is related to flow velocity. Others vary the current to maintain a fixed temperature, and thus the current required is related to flow velocity. Hot wire anemometers tend to break when used with liquids, so hot film probes are used in liquid applications. Venturi Meters: These flow-through meters measure the pressure drop across a restric tion and relate that to volume flow rate. The restriction is called a “throat” and has a grad - ual contraction followed by a gradual expansion. Venturi meters are more expensive than orifice meters, but cause less pressure loss in the system. Overall pressure loss is generally 5 to 20 percent of the measured differential pressure. Many venturi meters are available for measuring liquids, and others are meant for gases. They are suitable for contami- nated fluids and slurries. The lengths of straight pipe required upstream and downstream of a Venturi meter for accurate flow measurement are given in ISO 5167-1. Flow rate Q through a venturi is calculated using the following equation:
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