633 Measuring Steam and Condensate Rates

In maintaining steam systems at peak efficiency, it is often desirable to monitor the rate of steam flow through the system continuously, particularly at points of major

Fig. 6.4 Heat loss from steam leaks.

usage, such as steam mains. Figure 6.5 shows one of the most common types of flowmetering device, the calibrated orifice. This is a sharp-edged restriction that causes the steam flow to "neck down" and then re-expand after passing through the orifice. As the steam accelerates to pass through the restriction, its pressure drops, and this pressure drop, if measured, can be easily related to the flow rate. The calibrated orifice is one of a class of devices known as obstruction flowmeters, all of which work on the same principle of restricting the flow and producing a measurable pressure drop. Other types of obstruction meters are the ASME standard nozzle and the venturi. Orifices, although simple to manufacture and relatively easy to install, between flanges for example, are also subject to wear, which causes them, over a period of time, to give unreliable readings. Nozzles and venturis, although more expensive initially, tend to be more resistant to erosion and wear, and also produce less permanent pressure drop once the steam re-expands to fill the pipe. With all of these devices, care must be exercised in the installation, since turbulence and flow irregularities produced by valves, elbows, and fittings immediately upstream of the obstruction will produce erroneous readings.

Figure 6.6 shows another type of flowmetering device used for steam, called an annular averaging element. The annular element principle is somewhat different from the devices discussed above; it averages the pressure produced when steam impacts on the holes facing into the flow direction, and subtracts from this average impact pressure a static pressure sensed by a tube facing downstream. As with obstruction-type flowmeters, the flow must be related to this pressure difference.

A device that does not utilize pressure drop for steam metering applications is the vortex shedding flowmeter, illustrated in Figure 6.7. A solid bar extends through the flow, and as steam flows around the bar, vortices are shed alternately from one side to the other

Fig. 6.5 Orifice flowmeter.
Fig. 6.6 Annular averaging element.

in its wake, As the vortices shift from side to side, the frequency of shedding can be detected with a thermal or magnetic detector, and this frequency varies directly with the rate of flow. The vortex shedding meter is quite rugged, since the only function of the object extending into the flow stream is to provide an obstruction to generate vortices; hence it can be made of heavy-duty stainless steel. Also, vortex shedding meters tend to be relatively insensitive to variations in the steam properties, since they produce a pulsed output rather than an analog signal.

The target flowmeter, not shown, is also suitable for some steam applications. This type of meter uses a "target," such as a small cylinder, mounted on the end of a metal strut that extends into the flow line. The strut is gauged to measure the force on the target, and if the properties of the fluid are accurately known, this force can be related to flow velocity. The target meter is especially useful when only intermittent measurements are needed, as the unit can be "hot-tapped" in a pipe through a ball valve and withdrawn when not in use. The requirement of accurate property data limits the usefulness of this type of meter in situations where steam conditions vary considerably, especially where high moisture is present.

Fig. 6.7 Vortex shedding flowmeter with various methods for sensing fluctuations.

Weigh bucket technique for condensate measurement.

Fig. 6.7 Vortex shedding flowmeter with various methods for sensing fluctuations.

The devices discussed above are useful in permanent installations where it is desired to continuously or periodically measure steam flow; there is no simple way to directly measure steam flow on a spotcheck basis without cutting into the system. There is, however, a relatively simple indirect method, illustrated in Figure 6.8, for determining the rate of steam usage in systems with unpressurized condensate return lines, or open systems in which condensate is dumped to a drain. If a drain line is installed after the trap, condensate may be caught in a barrel and the weight of a sample measured over a given period of time. Precautions must be taken when using this technique to assure that the flash steam, generated when the condensate drops in pressure as it passes through the trap, does not bubble out of the barrel. This can represent both a safety hazard and an error in the measurements due to the loss of mass in vapor form. The barrel should be partially filled with cold water prior to the test so that flash steam will condense as it bubbles through the water. An energy balance can

Weigh bucket technique for condensate measurement.

also be made on the water at the beginning and end of the test by measuring its temperature, and with proper application of the steam tables, a check can be made to assure that the trap is not blowing through.

Figure 6.9 illustrates another instrument which can be used to monitor condensate flow on a regular basis, the rotameter. A rotameter indicates the flow rate of the liquid by the level of a specially shaped float which rises in a calibrated glass tube, such that its weight exactly balances the drag force of the flowing condensate.

Measurements of this type can be very useful in monitoring system performance, since any unusual change in steam or condensate rate not associated with a corresponding change in production rate would tend to indicate an equipment malfunction producing poor efficiency.

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