2000 Hook-up Book

Parallel and Series Operation of Reducing Valves

Bypasses The use of bypass lines and valves should usually be avoided. Where they are fitted, the capaci ty through the bypass should be added to that through the wide open reducing valve when sizing relief valves. Bypass valves are often found to be leaking steam because of wiredrawing of the seating faces when valves have not been closed tightly. If a genuine need exists for a bypass because it is essential to maintain the supply of steam, even when a reducing valve has developed some fault or is under going maintenance, considera tion should be given to fitting a reducing valve in the bypass line. Sometimes the use of a parallel reducing station of itself avoids the need for bypasses. Back Pressure Controls A Back Pressure regulator or sur plussing valve is a derivative of a pressure reducing valve, incorpo rating a reverse acting pilot valve. The pressure sensing pipe is con nected to the inlet piping so that the pilot valve responds to upstream pressure. Any increase in upstream pressure then opens the reverse acting pilot valve, causing the main valve to open, while a fall below the set pressure causes the main valve to close down, Fig. II-18 (page 92). These controls are useful in flash steam recovery applications when the supply of flash steam may at times exceed the demand for it. The BP control can then surplus to atmosphere any excess steam tending to increase the pressure within the flash steam recovery system, and maintains the recovery pressure at the required level. The control is also useful in eliminating non-essential loads in any system that suffers under capacity at peak load times, leaving essential loads on line. Back Pressure Controls are not Safety Valves and must never be used to replace them.

There is no hard and fast rule, but two valves in series will usually provide more accurate control. The second, or Low Pressure valve, should give the “fine control” with a modest turn down, with due consideration being given to valve sizes and capacities. A practical approach when selecting the turndown of each valve, that results in small est most economical valves, is to avoid having a non-critical drop in the final valve, and stay close to the recommended 10 to 1 turn down. Series Installations For correct operation of the valves, some volume between them is needed if stability is to be achieved. A length of 50 pipe diameters of the appropriately sized pipe for the intermediate pressure, or the equivalent vol ume of larger diameter pipe is often recommended. It is important that the down stream pressure sensing pipes are connected to a straight sec tion of pipe 10 diameters downstream from the nearest elbow, tee, valve or other obstruc tion. This sensing line should be pitched to drain away from the pressure pilot. If it is not possible to arrange for this and to still con nect into the top of the downstream pipe, the sensing line can often be connected to the side of the pipe instead. Equally, the pipe between the two reducing valves should always be drained through a stream trap, just as any riser downstream of the pressure reducing station should be drained. The same applies where a pressure reducing valve sup plies a control valve, and it is essential that the connecting pipe is drained upstream of the control valve.

This can be clarified by an example. Suppose that a maxi mum load of 5,000 lb/h at 30 psi can be supplied through one valve capable of passing 4,000 lb/h and a parallel valve capable of 1,300 lb/h. One valve is set at 29 psi and the other at 31 psi. If the smaller valve is the one set at 31 psi, this valve is used to meet loads from zero up to 1,300 lb/h with a controlled pressure at approximately 31 psi. At greater loads, the controlled pressure drops to 29 psi and the larger valve opens, until eventually it is passing 3,700 lb/h to add to the 1,300 lb/h coming through the smaller valve for a total of 5,000 lb/h. There may be applications where the load does not normally fall below the minimum capacity of the larger valve. It would then be quite normal to set the 4,000 lb/h valve at 31 psi and to supple ment the flow through the 1,300 lb/h valve at 29 psi in those few occasions when the extra capaci ty was required. Sometimes the split between the loads is effectively unknown. It is usual then to simply select valves with capacities of 1/3 and 2/3 of the maximum with the smaller valve at the slightly high er pressure and the larger one at the slightly lower pressure. Two-Stage or Series Operation Where the total reduction in pres sure is through a ratio of more than 10 to 1, consideration should be given to using two valves in series, Fig. 39 (page 20). Much will depend on the valves being used, on the total pressure reduction needed and the variations in the load. Pilot Operated controls have been used successfully with a pressure turndown ratio as great as 20 to 1, and could perhaps be used on a fairly steady load from 100 psig to 5 psi. The same valve would probably be unstable on a variable load, reducing from 40 to 2 psi.

SYSTEM DESIGN

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