2000 Hook-up Book

How to Size Temperature and Pressure Control Valves

Temperature Control Valve Sizing After estimating the amount of steam flow capacity (lbs/hr) which the valve must pass, decide on the pressure drop which can be allowed. Where the min imum pressure in a heater, which enables it to meet the load, is known, this value then becomes the downstream pressure for the control valve. Where it is not known, it is reasonable to take a pressure drop across the valve of some 25% of the absolute inlet pressure. Lower pressure drops down to 10% can give acceptable results where thermo-hydraulic con trol systems are used. Greater pressure drops can be used when it is known that the resulting down stream pressure is still sufficiently high. However, steam control valves cannot be selected with output pressures less than 58% of the absolute inlet pres

4. Correction for Superheated Steam The required Valve C v is the C v from the formula multiplied by the correction factor. Correction Factor = 1 + (.00065 x degrees F. superheat above saturation) Example: With 25°F of Superheat, Correction Factor = 1 + (.00065 x 25) = 1.01625 These provide a means of comparing the flow capac ities of valves of different sizes, type or manufacturer. The C v factor is determined experi mentally and gives the GPM of water that a valve will pass with a pressure drop of 1 psi. The C v required for a given application is estimated from the formu lae, and a valve is selected from the manufacturers catalog to have an equal or greater C v factor. sure. This pressure drop of 42% of the absolute pressure is called Critical Pressure Drop. The steam then reaches Critical or Sonic velocity. Increasing the pressure drop to give a final pressure below the Critical Pressure gives no further increase in flow. Pressure Reducing Valve Sizing Pressure reducing valves are selected in the same way, but here the reduced or downstream pressure will be specified. Capacity tables will list the Steam Flow Capacity (lb/h) through the valves with given upstream pressures, and varying downstream pres sures. Again, the maximum steam flow is reached at the Critical Pressure Drop and this value cannot be exceeded. It must be noted here that for self-acting regula tors, the published steam capacity is always given for a stated “Accuracy of Regulation” that differs among manufacturers and is not always the maxi mum the PRV will pass. Thus when sizing a safety valve, the C v must be used. Cv Values

SYSTEM DESIGN

1. For Liquids C v = GPM

Sp. Gr. √ Pressure Drop, psi

Where Sp. Gr. Water = 1 GPM = Gallons per minute

2. For Steam (Saturated) a. Critical Flow

When ∆ P is greater than F L 2 (P 1 /2) C v = W 1.83 F L P 1

b. Noncritical Flow

When ∆ P is less than F L 2 (P 1 /2) C v = W 2.1 √∆ P (P 1 + P 2 ) Where: P 1 = Inlet Pressure psia P 2 = Outlet Pressure psia W = Capacity lb/hr

F L = Pressure Recovery Factor

(.9 on globe pattern valves for flow to open) (.85 on globe pattern valves for flow to close)

5. Correction for Moisture Content Correction Factor = √ Dryness Fraction Example: With 4% moisture, Correction Factor = √ 1 - 0.04 = 0.98 6. Gas—Correction for Temperature Correction Factor = 460 + °F √ 520 Example: If gas temperature is 150°F, Correction Factor = 460 + 150 √ 520 = 1.083

3. For Air and Other Gases

a. When P 2 is 0.53 P 1 or less, C v = SCFH √ Sp. Gr. 30.5 P 1 Where Sp. Gr. of air is 1. SCFH is Cu. ft. Free Air per Hour at 14.7 psia, and 60°F. b. When P 2 is greater than 0.53 P 1 , C v = SCFH √ Sp. Gr. 61 √ (P 1 - P 2 ) P 2 Where Sp. Gr. of air is 1.

SCFH is Cu. Ft. Free Air per Hour at 14.7 psia, and 60°F.

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