2000 Hook-up Book

Temperature Control Valves for Liquid Service

Mixing And Diverting Three-Port Valves

another source, for use in show ers or similar open circuits where all the water does not recirculate, it is essential that the pressure of the supplies be equal. For these applications, it is recommended that both the X and Z ports be fit ted with check valves to prevent any scalding or other harmful back-flow condition. With the valve connected as shown in Fig. 43A, we now have a diverting arrangement. The valve has one inlet and two out lets. Hot water enters Port O and is either allowed through Port X to the equipment or through Port Z to return to the boiler. The Need for Balancing The action of a three-port valve in a closed circuit system, whether mixing or diverting, tends to change the pressure conditions around the system much less than does a two-port valve. This stabili ty is increased greatly when a balancing valve is fitted in the bypass (or mixing connection) line. Not fitting a flow balancing valve may result in short circuiting and starvation of other subcircuits. The balancing valve is set so that the resistance to flow in the bypass line equals or exceeds that in the load part of the subcircuit.

In Fig. 43, the balance valve must be set so that the resistance to flow in line B-Z is equal to the resistance to flow in line B-A-X. In Fig. 43A, resistance B-Z must equal resistance X-C-B. Makeup Air Heating Coils Air heating coils in vented con densate return systems, especially preheat coils supplied with low pressure steam modulat ed by a control valve, can present difficulties in achieving satisfacto ry drainage of condensate. There is no problem at full load with properly designed equipment, but part load conditions often lead to flooding of the coils with conden sate, followed by waterhammer, corrosion and sometimes by freeze-up damage. These prob lems are so widespread that it is worth examining their causes and remedies in some detail. Coil Configurations The coils themselves are usually built with a steam header and a condensate header joined by finned tubes. The headers may be both at one side of the unit, with hairpin or U tubes between them, or sometimes an internal steam tube is used to carry the steam to the remote end of an outer finned tube. Vertical headers may be used with horizontal finned tubes,

SYSTEM DESIGN

A three-port temperature control has one port that is constantly open and it is important that the circulating pump is always posi tioned on this side of the system. This will prevent the risk of pump ing against dead end conditions and allow correct circulation to do its job. The valve can be used either to mix or divert depending upon how it is piped into the system. A mixing valve has two inlets and one outlet, a diverting valve has one inlet and two outlets. Fig. 43 illustrates the three port valve used as a mixing valve in a closed circuit. It has two inlets (X and Z) and one outlet (O) which is the permanently open port. Port X is the port open on startup from cold, while Port Z will normally be closed on startup from cold. The amounts of opening in Ports X and Z will be varied to maintain a con stant outlet temperature from Port O. Thus a certain percentage of hot boiler flow water will enter through Port X to mix with a corre sponding percentage of cooler return water via Port Z. When the three-port valve is used to blend cold supply water with hot water which may be from

Case in Action: Hydrogen Compressor Cooling Jacket Temperature Control

Solution A 2" temperature control with adjustable bleed and a sens ing system was installed on the cooling water/Glycol outlet piping from three stages for each of two compressors. They were set to maintain a discharge temperature of 140°F. This had the effect of holding back Glycol in the jacket sufficiently to prevent excess hydrogen condensing while, at the same time, maintaining necessary cooling. Benefits • Reduced energy consumption as hydrogen condensing is reduced. • Installation of a self-contained control was far less expensive than a more sophisticated pneumatic type that was also under consideration. • System start-up was fast because of the easily-adjust ed, pre-calibrated sensing system. • Accurate process temperature control of each jacket resulted from having separate controls on each.

Hydrogen gas is an important ingredient to many oil refin ing processes. Large multi-stage compressors are located in operating sections throughout the refinery. Considerable attention is paid to maintaining gas quality, and keeping liquid from accumulating in the system. The telltale signs of entrained liquid became evident as a high-pitched whistling noise was heard coming from the compressor sections. It was determined to be the result of poor cooling water temperature control. The cool ing water/Glycol mixture leaving the heat exchanger at 95°F, circulating through the compressor jacket was caus ing excess hydrogen condensing on the cold surfaces of jacket walls. It’s important to maintain the 95°F heat exchanger outlet temperature to assure that sufficiently cool water/Glycol is supplied to the compressor sections necessary for proper heat transfer.

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