2000 Hook-up Book

Steam Meters

Density Compensation For accurate metering of com pressible fluids such as gases and vapors, the actual flowing density must be taken into account. This is especially true in the case of steam. If the actual flowing density of the steam is dif ferent to the specified density for which the meter was originally set up or calibrated, then errors will occur. These errors can be con siderable and depend on both the magnitude of difference between the specified density and the actual flowing density, and the type of meter being used. Example: The steam meter is set up and calibrated for 100 psig (specific volume = 3.89 ft 3 /lb) The steam is actually running at a pressure of 85 psig (specific vol ume = 4.44 ft 3 /lb). a. Differential Pressure Device (e.g. Orifice Plate or Gilflo Meter) Error = s.v. actual -1 x 100 √ s.v. specified Error = 4.44 -1 x 100 √ 3.89 Error = 6.8 Therefore the meter will over read by 6.8% [ [ [ (e.g. Vortex Meter) Error = s.v. actual -1 x 100 s.v. specified Error = 4.44 -1 x 100 3.89 Error = 14 Therefore the meter will over read by 14% In the case of saturated steam, pressure and temperature are related and therefore to establish the flowing density of saturated steam, either pressure or temper ature should be measured. In the case of superheated steam, pres sure and temperature can vary independently from one another and therefore to compensate for changes in density of superheat ed steam, both pressure and temperature must be measured. [ [ [ [ [ b. Velocity Device

Installation Ninety percent of all metering fail ures or problems are installation related. Care should be taken to ensure that not only is the meter selected suitable for the applica tion, but that the steam is correctly conditioned both to improve meter performance and provide a degree of protection, and that the manufacturer’s recommendations regarding installation are carefully followed. Steam Conditioning For accurate metering of saturat ed steam, irrespective of the meter type or manufacturer, it is important to condition the steam so that it is in the form of, or as close as possible to, a dry gas. This can be achieved by correct steam engineering and adequate

trapping to reduce the annular film of water that clings to the pipe wall, and effective separation ahead of the meter to remove much of the entrained droplets of water. It is therefore recommend ed that a steam conditioning station (as shown in Fig. 62) is positioned upstream of any type of meter on saturated steam applica tions. This will enhance accuracy and protect the meter from the effects of water droplets impacting at high velocity. Good steam engi neering such as the use of eccentric reducers, effective insu lation and adequate trapping will also prevent the dangerous effects of high velocity slugs of water known as waterhammer which can not only destroy meters but will also damage any valves or fit tings in it’s path.

SYSTEM DESIGN

Thermostatic Air Vent

Isolating Valve

Steam Separator

Steam to Meter

Check Valve

Strainer

Float & Thermostatic Steam Trap

Figure 62 Steam Conditioning Station

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