The Engineer of the Toffee Factory and I were discussing the merits of using a Pilot Operated Steam Pressure Reducing Valve, (PRV). "So for accurate control of Steam Pressure, I just need to use a Pilot Operated PRV," stated the Engineer, "or should I go for sophistication with electronic control?"

I hesitated in my reply as I have been asked this question so many times and knew that the answer was far from being straightforward. "Strangely enough," I responded, "much more important than sophistication is the attention to the detail regarding the installation of whichever type of Reducing Valve you choose. Allow me to explain."

I then outlined the decisions facing him if he was to ensure accurate control of pressure.

The choice between a self operated Pilot PRV, or for example a Pneumatic Control Valve with Electronic Controller and Pressure sensors, was in truth dependent upon the Steam Process involved. If regular changes in pressure were required in order to adjust for each specific product being manufactured, then a sophisticated Control, such as a Pneumatic Valve, would be a good choice, as the electronic controller could be programmed accordingly. However, if the Steam Pressure to be supplied to the process was required to be maintained at a constant set pressure, then a Pilot PRV would be less costly to install and work with surprising accuracy, providing other criteria were met. In fact, the same criteria would apply to both forms of Control Valves.

"So what must I do to ensure accurate control and long reliable service from the PRV I install?" asked the Engineer. I listed them as follows, though not necessarily in any order of priority and then went on to offer a brief explanation on each factor involved:

1. Pipe Sizing

2. Turn down ration

3. Wet steam prevention

4. Internal versus external sensing

5. Sensing point

6. Turbulence in secondary piping

7. Steam trap drainage of the Steam Mains

1. Pipe Sizing

Without doubt this is critical and is necessary to ensure that the Steam velocity is kept low, which will also reduce the wear on the PRV and so lower maintenance costs. Correctly sized Steam Pipes smooth out turbulence in Steam flow, which in turn means that the pressure being sensed by the PRV is not fluctuating. (See attached PDF on Steam Pipe Sizing)

2. Turn Down Ratio

The majority of Manufacturers of PRV's publish capacity charts for their valves and in some instances stipulate a maximum Turn Down Ratio. That is the amount of pressure reduction the valve can handle without it becoming slightly unstable and hence giving erratic control. A typical example would be a Turn Down Ratio of 10 to 1, so for example a PRV could reduce from 100psig to 10 psig without a problem, but below this would become erratic. It is also a fact that too high a Turn Down will lead to a higher level of noise in the piping, with then a requirement for silencers.

3. Wet Steam Prevention

The quality of steam flowing through the piping system will vary according to several factors, but in the majority of Factories is certainly not totally dry saturated steam as it reaches the Control Valves or PRV. In fact it will carry entrained moisture and as such presents a potential hazard to any Valve in its path and certainly is a contributory factor to rapid wear of PRV Valve components. A hazard, because a slug of condensate picked up by the steam as it flows along the pipe, can physically damage Valves in its path. Wet Steam will, over a period of time, also "wire draw" valve parts leading to early failure. The solution to this problem is to install a separator prior to the PRV.

4. Internal versus External Sensing

Many self acting Pilot Operated PRV's have the option of selecting internal sensing, or external sensing via a pipe connection to the downstream secondary pipe.

see Figure 8 - Installation of PRV (Related PDF below)


Internal sensing simplifies the installation, but can lead to a reduction in capacity, that is the amount of steam that can pass through it, of that PRV. One manufacturer of such Valves states that this is because at the outlet throat of the PRV where the secondary pressure is sensed, the steam flow is quite turbulent, which in turn gives rise to a localized higher pressure than that shown on a gauge fitted some further distance downstream to the PRV. Hence, the Valve closes sooner than it would if the pressure sensing was external. If capacity is important, then external sensing must be used.

5. Sensing Point

Without doubt, external sensing is to be preferred, see Figure 8, as if the sensing point is situated in a clear length of pipe with 15 pipe diameters clear pipe either side of the sensing point, then smooth flow condition exist within the pipe and as such the PRV does not constantly have to try and readjust to fluctuations occurring due to turbulent flow within the pipe. In order to ensure a fall in the sensor pipe to the steam main itself, the sensor pipe should be connected not into the top of the stem main, but at a position of 3 or 9 o'clock. That way condensate cannot lie in the sensor pipe and cause erratic operation.

6. Turbulence in secondary piping

Other causes of turbulence in the steam flow downstream of the PRV can be due to the pipe configuration. Sudden sharp bends close to the PRV, or take offs to Branch Mains soon after the PRV, will all lead to small changes in pressure within the pipe, so that the PRV is constantly readjusting itself and giving the appearance that the PRV is unstable.

7. Steam Trap drainage of the Steam Mains

Many Engineers do try and ensure that the Steam Mains are drained of condensate either side of any PRV, by installing Steam Traps at those points. However in some instances they choose to fit Steam Traps that have a blast discharge action, such as a Thermodynamic, or Inverted Bucket type. Unfortunately these are not ideal for this application for when such traps open to discharge they will cause a momentary change in pressure conditions close to any PRV. If the PRV is a relatively small size of valve, say up to 2" in size, then it will sense the sudden change and attempt to react, so causing small variations in the accuracy of control. The first choice of Steam Trap for such applications is a Float Type, which will have a smooth discharge of condensate.

Summary

For Reduction of Steam Pressure to one piece of equipment, where pressure and temperature are not critical, then a basic Simple Pressure Reducing Valve is quite adequate. Where a number of items of plant are being supplied with Steam, then a pilot Operated PRV is the first choice. To ensure accurate control of Steam Pressure then a good deal of time should be invested in designing the installation.


For further information or advice, contact Control Specialties.