Honeywell MS4103 0000240 C, 865 C, Pascals, Meter, ∆p k vs= Static pressure loss of 10 5 pascals

Appendix A: Valve Selection and Sizing

Valve Flow Terms

Rangeability: The ratio of maximum flow to minimum controllable flow. Approximate rangeability ratios are 50 to 1 for V-port globe valves and 30 to 1 for contoured plug valves.

EXAMPLE:

A valve with a total flow capacity of 100 gpm full open and a rangeability of 30 to 1, can accurately controls flow accurately as low as 3 gpm.

Tight shut-off/close-off: A valve condition in which virtually no leakage of the controlled fluid occurs in the closed position. Generally, only single-seated valves provide tight shut-off. Double-seated valves typically have a one to three percent leakage in the closed position.

Turndown: The ratio of maximum flow to minimum controllable flow of a valve installed in a system. Turndown is equal to or less than rangeability.

EXAMPLE:

For the valve in the rangeability example, if the system requires a 66 gpm maximum flow through the valve and since the minimum accurately controllable flow is 3 gpm, the turndown is 22.

Valve Ratings

Flow coefficient (capacity index): Used to state the flow capacity of a control valve for specified conditions. In the control valve industry currently one of three flow coefficients is used depending upon the location and system of units; British Av, European kvs, or United States Cv . The flow coefficients have the following relationships:

Av	=	0.0000278 kvs
Av	=	0.0000240 Cv
kvs	=	0.865 Cv

The flow coefficient Av is in cubic meters per second and can be determined from the formula:

A		p
	V	= Q -------
		∆p

Where:

Q = volumetric flow in cubic meters per second.

ρ= fluid density in kilograms per cubic meter.

∆p	=	static pressure loss across the valve in
		pascals.

The flow coefficient kvs is water flow in cubic meters per hour with a static pressure loss across the valve of

105 pascals (1 bar) within the temperature range of 5 to 40°C and can be determined from the formula:

		k		= Q	∆ρkvs	ρ
			vs		----------------	• -------
Where:					∆ρ	ρw
Q	=	volumetric flow in cubic meters per hour.

ρ= fluid density in kilograms per cubic meter.

ρw	=	density of water in kilograms per cubic
		meter.
∆p kvs=		static pressure loss of 105 pascals.
∆p	=	static pressure loss across the valve in
		pascals.

The flow coefficient Cv is water flow in gallons per minute with a pressure loss across the valve of one pound per square inch within the temperature range of 40 to 100F and can be determined for other conditions from the formula:

		C		= Q	1	ρ
			V		------	• -------
Where:					∆ρ	ρw
Q	=	volumetric flow in US gallons per minute.

ρ= fluid density in pounds per cubic foot.

ρw	=	density of water in pounds per cubic foot
		within the temperature range of 40
		to 100F
∆p	=	static pressure loss across the valve in
		pounds per square inch.

Close-off rating: The maximum pressure drop that a valve can withstand without leakage while in the full closed position. The close-off rating is a function of actuator power to hold the valve closed against pressure drop, by structural parts such as the stem can be the limiting factor. The construction of gate-style valves, such as ball valves, often allows them to hold back high head pressures in the closed position, although the actuator may not be powerful enough to operate the valve against such forces.

EXAMPLE:

A valve with a close-off rating of 10 psi could have 40 psi upstream pressure and 30 psi downstream pressure. Note that in applications where failure of the valve to close is hazardous, the maximum upstream pressure must not exceed the valve close-off rating, regardless of the downstream pressure.

The valve close-off rating is independent of the actual valve body rating. See definition of BODY RATING (ACTUAL).

APPENDIX

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