2000 Hook-up Book

Temperature Control Valves for Steam Service

Temperature Control Valves For Steam Service As with pressure reducing valves, temperature control valves can be divided into three groups. Installation of these valves are the same as pressure reducing styles in that adequate protection from dirt and condensate must be used as well as stop valves for shutdown during maintenance procedures. A noise diffuser and/or a safety valve would nor mally not be used unless a combination pressure reducing and temperature control, is installed. See PRV station com ponents on page 20 for more information. Direct Operated Valves The direct operated type as shown in Fig. 40 are simple in design and operation. In these controls, the thrust pin movement is the direct result of a change in temperature at the sensor. This movement is transferred through the capillary system to the valve, thereby modulating the steam flow. These valves may also be used with hot water. Such a sim ple relationship between

temperature changes and valve stem movement enables sensor and valve combinations to give predictable valve capacities for a range of temperature changes. This allows a valve to be selected to operate with a throttling band within the maximum load propor tional band. See appropriate technical sheets for specific valve proportional bands. Choice of Proportional Band is a combination of accuracy and stability related to each applica tion. However, as control accuracy is of primary impor tance, and as direct operated controls give constant feedback plus minute movement, we can concentrate on accuracy, leaving the controller to look after stabili ty. Generally, to give light load stability, we would not select a Proportional Band below 2°F. Table 9 gives the span of accept able Proportional Bands for some common heat exchanger applica tions. Pilot Operated Valves Greater steam capacities are obtained using pilot operated valves, along with greater accura

cy due to their 6°F proportional band. Only a small amount of steam has to flow through the pilot to actuate the main diaphragm and fully open the valve. Only very small changes of movement within the sensor are necessary to produce large changes in flow. This results in accurate control even if the upstream steam pres sure fluctuates. Both direct and pilot operated valve types are self-contained and do not require an external power source to operate. Table 9 Acceptable Proportional Bands for Some Common Applications Proportional Application Band °F Domestic Hot Water 7-14 Heat Exchanger Central Hot Water 4-7 Space Heating 2-5 (Coils, Convectors, Radiators, etc.) Bulk Storage 4-18 Plating Tanks 4-11

SYSTEM DESIGN

Case in Action: Dry Coating Process

An office paper product manufacturer uses steam in its process for a dry coating applied to the paper. Using a pocket ventilation system, air is blown across the paper as it moves through the dryer cans. The original design included inverted bucket type traps on the outlet of the steam coils, but the coils are in overload boxes where outdoor and indoor air mix. The steam supply is on a modulating control with maximum pressure of 150 psi. The steam traps discharge into a com mon header that feeds to a liquid mover pump. The pump had a safety relief valve on its non-vented receiver. Problems observed included the inability to maintain desired air temperatures across the machines, high back pressure on the condensate return system, the doors on the coil boxes had to be opened to increase air flows across the coils, paper machine had to be be slowed down to improve dryness, steam consumption was way up, water make-up was up and vent lines were blowing live steam to the atmosphere. Solution: A pump trap combination was installed on five of the nine sections using a pressure regulator for motive steam sup

ply reduction to the pumps. Float & thermostatic traps with leak detection devices were also installed for efficiency. Closed doors were then put on the coil boxes. The back pressure on the return system dropped to an acceptable and reasonable pressure and the steam con sumption also dropped. Temperature control was achieved and maintained and production increased from 1,000 feet per minute on some products to 1,600 feet per minute. They switched all five sections of the paper coater to 1" low profile Pressure Powered Pump with cast iron float & ther mostatic steam traps. This manufacturer also switched from inverted buckets on heating units to float and thermo static steam traps with leak detection devices and replaced several electric pumps and the liquid mover with Pressure Powered Pumps. Replaced all 16 inverted bucket traps on paper coater with float and thermostatic steam traps with leak detection devices. Benefits: • Production Increased • Trap failure went from 40% to 14%. • Over a half-million dollars in steam saved during first year of operation

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