Valve capacity

Manufacturers of valves, especially control valves, express valve capacity in terms of a flow coefficient C,, which gives the flow rate through the valve in gal/min of water at 60°F under a pressure drop of 1 IbFin. It is related to K by... 

Safety Relief Valves Conventional safety relier valves (Fig. 26-14) are used in systems where built-up backpressures typically do not exceed 10 percent of the set pressure. The spring setting or the valve is reduced by the amount of superimposed backpressure expecied. Higher built-up backpressures can result in a complete loss of continuous valve capacity. The designer must examine the effects of other relieving devices connected to a common header on the performance of each valve. Some mechanical considerations of conventional relief valves are presented in the ASME code however, the manufacturer should be consulted for specific details. 

Refrigerant circulation or kettle or Chiller 2. Expansion valve capacity. 

The quantity of material to be relieved should be determined at conditions corresponding to the PR valve set pressure plus overpressure, not at normal operating conditions. Frequently, there is an appreciable reduction in required PR valve capacity when this difference in conditions is considerable. The effect of friction pressure drop in the connecting line between the source of overpressure and the system being protected should also be considered in determining the capacity requirement. If the valve passes a liquid which flashes or the heat content causes vaporization of liquid, this should be considered in determining PR valve size. 

PR valve capacity should be equal to the compressor capacity at the emergency conditions. For centrifugal compressors, the combination of PR valve set point and relieving capacity should be such as to avoid surge conditions over... 

In addition to the above back pressure limitations based on valve capacity, balanced bellows PR valves are also subject to back pressure limitations based on the mechanical strength of the bellows or bellows bonnet, or the valve outlet flange rating. The back pressure specified for the valve is governed by the lowest back pressure permitted by these various criteria. 

In general, the total back pressure on a balanced bellows pressure relief valve (superimposed plus built-up) should be limited to 50% of set pressure, because of the marked effect of higher back pressures on valve capacity, even when appropriate correction factors are used in sizing. In exceptional cases, such as a balanced bellows PR valve discharging into another vessel, total pressure up to 70% of set pressure may be used. 

It is important to note that back pressure affects balanced PR valve capacities in the same way as for conventional valves, and appropriate factors are included in the sizing procedures. They are subject to the same recommended limits of maximum total back pressure (superimposed plus built-up) as conventional valves. In the case of balanced bellows valves, mechanical considerations must also be evaluated, since they may limit the maximum permissible back pressure. 

Pilot-operated valves have the advantage of allowing operations n the set point v/ith no leakage, and the set position is not affected by ba pressure. However, they will not function if the pilot fails. If the sens line fills with hydrates or solids, the valve will open at 25% over pressure trapped above the disc (usually the normal operating pre the vessel). For this reason they should be used with care in dirty vice and liquid service. They are used extensively offshore where all platform relief valves are tied into a single header because up to 5 back-pressure will not affect the valve capacity. 

All relief valves are affected by reaching critical flow, which corre-spond.s to a back-pressure of about 50% of the set pressure. Pilot-operated relief valves can handle up to 50% back-pressure without any significant effect on valve capacity. Back-pressure correction factors can be obtained from the relief valve manufacturers for back-pre.ssures above 50%. API RP 520 gives a generic method for sizing a pilot-operated relief valve for sub-critical flow. 

Effects of Two-Phase Vapor-Liquid Mixture on Relief Valve Capacity... 

Nye Tray, 10-20% increased tray (over sieve or valve) capacity and good efficiency. More capacity from existing column. Improved inlet area for sieve or valve tray with greater area for vapor-liquid disengagement. 

Valve capacities can be compared by use of the Kv (or Cv when Imperial units are used) values. These factors are determined experimentally, and the Kv value is the number of cubic meters per hour of water that will flow through a valve with a pressure drop of one bar. The Cv value is the number of gallons per minute of water that will flow through the valve with a pressure drop of one-pound f. per square inch. As the gallon is a smaller unit in the USA, the number of gallons passed is greater, and the US Cv is 1.2 times the UK Cv. The Kv is about 0.97 of the UK Cv value. 

W = Required valve capacity in lbs/h V = Latent heat of evaporization in BTU/lb... 

A balanced piston SRV can handle all applications mentioned for balanced bellows SRVs, but should the vulnerable bellows fail, the piston on top of the guide maintains the proper performance of the valve, with no change in opening pressure and no reduction in valve capacity. The balanced piston ensures a stable valve performance. 

While this is the general recommendation, compliance with this is not always possible. As seen earlier in this book, higher oudet pressure drops may result in reduced valve capacity and instability. In that case, other types of SRVs as discussed in Chapter 9 need to be taken into consideration. API RP 521 also states the outlet pressure drop for individual relief valves should be based on the actual rated valve capacity consistent with the inlet piping pressure drop as discussed in Section 6.1. 

Control valves. Valve vendors publish much information about flow-control valves. Much of their information covers what is known as control valve flow coefficients Cv. It is convenient to express the valve capacity and the valve flow characteristics in terms of the flow coefficient Cv. This value is defined as the flow of water at 60°F, in gallons per minute (gpm), at a pressure drop of 1.0 psi across the valve. In a flow valve, therefore, controlling the flow of water at 60°F as the valve opens, keeping a 1.0-psi pressure drop across the valve, the C value is the rate of water flow, gpm. 

A control valve can also be viewed as variable-area flow meter. Therefore, smart valves can measure their own flow by solving the appropriate valvesizing equation. For example, in the case of turbulent liquid flow applications, where the valve capacity coefficient... 

Representative Values of Relative Valve Capacity Coefficients (Cd) and of Other Sizing Factors for a Variety of Valve Designs. The Cd Values Listed Are for Valves with Full Area Trims, When the Valve Is Fully Open... 

The valve body size is frequently determined not only on the basis of the required valve capacity (C ), but also by considering the maximum acceptable flow velocity (approximately 30 ft/s [9 m/s]). 

Select the diaphragm-actuated valve size. Using a manufacturer s engineering data for an acceptable valve, enter the appropriate valve-capacity table at the valve inlet steam pressure, 225 psig, and project to a capacity of 500 lb/h, as in Table 6.16. Read the valve size as 3/4 in at the top of the capacity column. 

Determine the valve capacity required. Pressure-reducing valves in water systems operate best when the nominal load is 60 to 70 percent of the maximum load. Using 60 percent, the maximum load for this valve = 1200/0.6 = 2000 gal/h (2.1 L/s). 

Determine the valve size required. Enter a valve-capacity table in suitable valve engineering data at the valve inlet pressure and project to the exact, or next higher, valve capacity. Thus, enter Table 6.19 at 140 lb/in2 (965.2 kPa) and project to the next higher capacity, 2200 gal/h (2.3 L/s), since... 

Some valve manufacturers present the capacity of their valves in graphic instead of tabular form. One popular chart, Fig. 6.9, is entered at the difference between the inlet and outlet pressures on the abscissa, or 140 — 40 = 100 lb/in2 (689.4 kPa). Project vertically to the flow rate of 2000/60 = 33.3 gal/min (2.1 L/s). Read the valve size on the intersecting valve-capacity curve, or on the next curve if there is no intersection with the curve. Figure 6.9 shows that a 1-in valve should be used. This agrees with the tabulated capacity. 

Classes II, III and IV call for progressively tighter shutoff, expressed as a percentage of valve capacity. Classes V and VI are a bit more complicated (see the table). 

Refrigerant circulation or kettle level (possible inadequate flow resulting in superheating of refrigerant). 4. Process overload of refrigerant system. or Chiller 2. Expansion valve capacity. 3. Chiller or economizer level control malfunction. 4. Restriction in refrigerant flow (hydrates or ice). 

Flow-through relief valves have traditionally been analyzed by treating the valve as a convergent noz-2ie [i 29.30] discharge coefficient (kactual mass flow rate to the theoretical ideal mass flow rate through a one-dimensional convergent nozzle of exit area equal to the nominal bore of the tested valve. Measured discharge coefficients are derated by 10% (Aidr) to specify valve capacity. 

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