Honeywell MS4103 Negative value if a vacuum return, Psig, Pressure or 0 psig, Pavg =, 260

Appendix A: Valve Selection and Sizing

2.Determine the critical pressure drop:

		hcritical = 50% x Pma
Where:
Pma	=	Pressure in supply main in psia (absolute
		pressure)
psia	=	psig + 14.7

Use the smaller value h or hcritical when calculating Cv.

Two-Position Applications

Use line sized valves whenever possible. If the valve size must be reduced, use:

		h = 20% x (Pm-Pr)
Where
Pm	=	Pressure in supply main in psig or psia
		(gage or absolute pressure).
Pr	=	Pressure in return in psig or psia. A
		negative value if a vacuum return.

STEAM VALVE SIZING EXAMPLES

EXAMPLE 1:

A two-way linear valve (V1) is needed to control high-pres- sure steam flow to a steam-to-water heat exchanger. An industrial-type valve is specified. Steam pressure in the sup- ply main is 80 psig with no superheat, pressure in return is equal to atmospheric pressure, water flow is 82.5 gpm, and the water temperature difference is 20F.

Use the steam valve Cv formula to determine capacity index for Valve V1 as follows:

(1 + 0.00075s)Q V Cv = -------------------------------------------------

63.5 h

Where:

Q= The quantity of steam required to pass through the valve is found using the converter valve formula:

Q = gpm ⋅ TDw ⋅ 0.49

Where:

gpm = 82.5 gpm water flow through exchanger TDw = 20F temperature difference

0.49 = A scaling constant

Substituting this data in the formula:

Q = 808.5 pounds per hour

h= The pressure drop across a valve in a modulating application is:

		h = 85% x (Pm-Pr)
Where:
Pm	=	Upstream pressure in supply main is 80
		psig.
Pr	=	Pressure in return is atmospheric
		pressure or 0 psig.

Substituting this data in the pressure drop formula:

h= 0.80 x (80 – 0)

=0.80 x 80

=64 psi

The critical pressure drop is found using the following for- mula:

hcritical = 50% x (psig + 14.7 psi)

hcritical= 0.50 x (80 psig upstream + 14.7 psi)

=0.50 x 94.7 psi

=47.4 psi

The critical pressure drop (hcritical) of 47.4 psi is used in cal culating Cv, since it is less than the pressure drop (h) of 64 psi. Always, use the smaller of the two calculated values.

V= Specific volume (V) of steam, in cubic feetper pound at average pressure in

	valve (Pavg):
Pavg =	h
	Pm – --
	2
=	47.4
	80 – ---------= 80 – 23.6 = 56.4psig
	2

The specific volume of steam at 56.4 psig is 6.14 and the square root is 2.48.

63.5 = A scaling constant.

Substituting the quantity of steam, specific volume of steam, and pressure drop in the Cv formula shows that the valve should have a Cv of 4.6.

Cv = (---1----+-----	0.00075---------------------⋅-----0-----)--⋅-----808.5---------------⋅-----2.48--------
	63.5	47.4
=	1745.6	= 4.6
	63.5-------------⋅----6.88---------

NOTE: If Pavg is rounded off to the nearest value in Table 5 (60 psi), the calculated Cv is 4.5 a negli- gible difference.

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