Closing Characteristics of Valves

Depending on the body and piston for a type of valve, closing it by moving the piston at a constant rate results in a different rate of decrease in the area open to flow. Near the end of the closure, some types decrease this area faster while others slow down. Bentley HAMMER CONNECT has built-in area-closure characteristics for various types of valves to ensure this important factor is represented adequately. You can select the correct valve type and know that the decrease in flow will be modeled in a realistic manner as the valve closes.

Note: For most manufacturers, the rate at which area decreases as the valve closes is a close approximation to the rate at which flow decreases, often reported as a C_v curve. If either curve is available for your valve, you can enter it as an area-closure curve in Bentley HAMMER CONNECT.

For ease of interpretation, valve closing can be represented numerically by the shape of closure (S) parameter that represents the rate of opening area deceleration during the time of a complete closure (T_c), or stroke time, if the stroke varies linearly with time. If a partial closure, opening, or full opening is specified, Bentley HAMMER CONNECT correctly tracks the area open to flow. The following equations are used to relate area to stroke:

Increasing deceleration—If the rate of change of the area open to flow (with respect to a constant stroke speed) increases at the end of the closure period, the valve closing pattern can be expressed as:

A/A ₀ = 1 - (T/Tc ) ^-S

Where:	A/A₀	=	the fraction of the full valve-opening area
	T/T_c	=	the fraction of time required to completely close the valve
	S	=	the shape of valve closure, which is greater than 1 for increasing deceleration

Decreasing deceleration—If the rate of change of the area open to flow (with respect to a constant stroke speed) decreases at the end of the closure period, the exponent S should be less than 1 and the valve-closing pattern can be expressed as
A/A ₀ = (1 - T/Tc ) ^- S

For valves commonly used in engineering practice, the following values of S are used by Bentley HAMMER CONNECT according to the valve type:

Valve	S
Butterfly valve	-1.85
Ball valve	-1.35
Globe valve	1.00
Circular gate valve	1.35
Needle valve	2.00
User-defined (enter curve)	n/a

The relationship between the fraction of area open to flow (A/A₀) and the stroke (T/T_c) is shown in the following figure.

Figure 14-9: Relationship between Fraction of Area Open to Flow and Stroke