2000 Hook-up Book

Pressure Reducing Stations

Pressure Reducing Stations It is a mistake to install even the best of pressure reducing valves in a pipeline without giving some thought to how best it can be helped to give optimal perfor mance. The valve selected should be of such a size that it can handle the necessary load, but oversiz ing should be avoided. The weight of steam to be handled in a given time must be calculated or estimated, and a valve capable of passing this weight from the given upstream pressure to the required downstream pressure is chosen. The valve size is usually smaller than the steam pipes either upstream or downstream, because of the high velocities which accompany the pressure drop within the valve. Types of Pressure Reducing Valves are also important and can be divided into three groups The direct acting valve shown in Fig. II-17 (page 91) is the sim plest design of reducing valve. This type of valve has two drawbacks in that it allows greater fluctuation of the down stream pressure under unstable load demands, and these valves have relatively low capacity for their size. It is nevertheless per fectly adequate for a whole range of simple applications where accurate control is not essential and where the steam flow is fairly small and constant. Pilot Operated Valves Where accurate control of pres sure or large capacity is required, a pilot operated reducing valve should be used. Such a valve is shown in Fig. II-12 (page 89). The pilot operated design offers a number of advantages over the direct acting valve. Only a very small amount of steam has to flow through the pilot valve to pres surize the main diaphragm of operation as follows: Direct Operated Valves

chamber and fully open the main valve. Thus, only very small changes in downstream pressure are necessary to produce large changes in flow. The “droop” of pilot operated valves is therefore small. Although any rise in upstream pressure will apply an increased closing force on the main valve, this is offset by the force of the upstream pressure acting on the main diaphragm. The result is a valve which gives close control of downstream pressure regardless of variations on the upstream side. Pneumatically Operated Valves Pneumatically operated control valves, Fig. II-20 (page 93), with actuators and positioners being piloted by controllers, will provide pressure reduction with even more accurate control. Controllers sense down stream pressure fluctuations, interpolate the signals and regu late an air supply signal to a pneumatic positioner which in turn supplies air to a disphragm open ing a valve. Springs are utilized as an opposing force causing the valves to close upon a loss or reduction of air pressure applied on the diaphragm. Industry sophistication and control needs are demanding closer and more accurate control of steam pres sures, making pneumatic control valves much more popular today. Piping And Noise Consideration The piping around a steam pres sure reducing valve must be properly sized and fitted for best operation. Noise level of a reduc ing station is lowest when the valve is installed as follows: 1. Avoid abrupt changes in direction of flow. Use long radius bends and “Y” piping instead of “T” connections. 2. Limit approach and exit steam velocity to 4000 to 6000 FPM.

3. Change piping gradually before and after the valve with tapered expanders, or change pipe only 1 or 2 sizes at a time. 4. Provide long, straight, full-size runs of heavy wall pipe on both sides of the valve, and between two-stage reductions to stabilize the flow. 5. Use low pressure turndown ratios (non-critical.) 6. Install vibration absorbing pipe hangers and acoustical insulation. Most noise is generated by a reducing valve that operates at critical pressure drop, especially with high flow requirements. Fitting a noise diffuser directly to the valve outlet will reduce the noise level by approx. 15 dBA. It must also be remembered that a valve designed to operate on steam should not be expected to work at its best when supplied with a mixture of steam, water and dirt. A separator, drained with a steam trap, will remove almost all the water from the steam entering the pressure reducing set. The baffle type separator illustrated in Fig. 36 has been found to be very effective over a broad range of flows.

SYSTEM DESIGN

Figure 36 Moisture Separator for Steam or Air

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