2000 Hook-up Book

Parallel and Series Operation of Reducing Valves

Case in Action: Elimination of Steam Energy Waste

As part of a broad scope strategy to reduce operating costs throughout the refinery, a plan was established to eliminate all possible steam waste. The focus of the plan was piping leaks, steam trap failures and steam pressure optimization. Programs having been previously established to detect/repair steam trap failures and fix piping leaks, par ticular emphasis was placed on steam pressure optimization. Results from a system audit showed that a considerable amount of non-critical, low temperature trac ing was being done with 190 psi (medium pressure) steam, an expensive overkill. It appeared that the medium pressure header had been tapped for numerous small tracing projects over the years. Solution Refinery engineers looked for ways to reduce pres sure to the tracer lines. Being part of a cost-cutting exercise, it had to be done without spending large sums of capital money on expensive control valves. The self-con

tained cast steel pressure regulators and bronze reducing valves were chosen for the job. In 1-1/2 years, approxi mately 40 pressure regulators and hundreds of bronze reducing valves have been installed at a cost of $250K. Annualized steam energy savings are $1.2M/year. More specifically, in the Blending and Shipping Division, $62,640 was saved during the winter of 1995, compared to the same period in 1994. Benefits • Low installed cost. The Spirax Sarco regulators and bronze reducing valves are completely self-contained, requiring no auxiliary controllers, positioners, convert ers, etc. • Energy savings worth an estimated $1.2M/year. • The utilities supervisor who worked closely with Spirax Sarco and drove the project through to successful com pletion received company wide recognition and a promotion in grade.

SYSTEM DESIGN

Parallel Operation In steam systems where load demands fluctuate through a wide range, multiple pressure control valves with combined capacities meeting the maximum load per form better than a single, large valve. Maintenance needs, down time and overall lifetime cost can all be minimized with this arrange ment, Fig. 38 (page 20). Any reducing valve must be capable of both meeting its maxi mum load and also modulating down towards zero loads when required. The amount of load turndown which a given valve can satisfactorily cover is limited, and while there are no rules which apply without exception, if the low load condition represents 10% or less of the maximum load, two valves should always be pre ferred. Consider a valve which moves away from the seat by 0.1 inches when a downstream pres sure 1 psi below the set pressure is detected, and which then pass es 1,000 pounds per hour of steam. A rise of 0.1 psi in the detected pressure then moves the valve 0.01 inches toward the

seat and reduces the flow by approximately 100 pph, or 10%. The same valve might later be on a light load of 100 pph total when it will be only 0.01 inches away from the seat. A similar rise in the downstream pressure of 0.1 psi would then close the valve completely and the change in flow through the valve which was 10% at the high load, is now 100% at low load. The figures chosen are arbitrary, but the prin ciple remains true that instability or “hunting” is much more likely on a valve asked to cope with a high turndown in load. A single valve, when used in this way, tends to open and close, or at least move further open and further closed, on light loads. This action leads to wear on both the seating and guiding surfaces and reduces the life of the diaphragms which operate the valve. The situation is worsened with those valves which use pis tons sliding within cylinders to position the valve head. Friction and sticking between the sliding surfaces mean that the valve head can only be moved in a

series of discreet steps. Especially at light loads, such movements are likely to result in changes in flow rate which are grossly in excess of the load changes which initiate them. Load turndown ratios with piston operated valves are almost inevitably smaller than where diaphragm-operated valves are chosen. Automatic selection of the valve or valves needed to meet given load conditions is readily achieved by setting the valves to control at pressures separated by one or two psi. At full load, or loads not too much below full load, both valves are in use. As the load is reduced, the controlled pressure begins to increase and the valve set at the lower pres sure modulates toward the closed position. When the load can be supplied completely by the valve set at the higher pressure, the other valve closes and with any further load reduction, the valve still in use modulates through its own proportional band. Pressure Settings for Parallel Valves

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