Control valves, such as pressure reducing valves (PRV), modify their opening to control pressure or flow in the system. For example, PRV's adjust valve position to reduce inlet pressure meet a target outlet pressure.

Through HAMMER V8i SELECT series 3, HAMMER maintained a constant valve position throughout a transient analysis. In many cases that opening is correct, but there are instances where the valve position will modulate significantly in response to the transient and must be accounted for. In some instances, valve modulation can contribute to transient problems.

With SELECT series 4, there is a new PRV property "Modulate Valve during Transient" which, when set to True, enables HAMMER to adjust the valve opening during a transient run. The default value for this property is False. This property is saved in the Transient alternative.

When "Modulate Valve during Transient" is set to True, the user must set the "Opening rate coefficient" and Closure rate coefficient". The units for these properties are % change in opening/second/foot of HGL difference between the control valve setting and the calculated pressure at the previous time step (xxx %/sec/ft or yyy %/sec/m). These values are highly valve specific. The default values are for both rates. The closing and opening rates for a given valve may be different. Values will be lower for larger valves and will be much higher for direct acting valves than pilot controlled valves. The values should be calibrated using high speed pressure loggers. A reasonable initial estimate may be on the order of 0.1.

The valve position is calculated in HAMMER as

V(t+1) = V(t) + cr (H(t) - Hs) dt, if H(t) > Hs

V(t+1) = V(t) + co (H(t) - Hs) dt, if H(t) < Hs

Where:

• V= valve position (% closed)
• cr = closing rate (%/s/ft)
• cr = opening rate (%/s/ft)
• Hs = target outlet hydraulic grade (ft)
• H(t) = outlet hydraulic grade at time t (ft)
• dt = time step size, s

If the opening or closing rates are set too high, it is possible to create numerical instability in HAMMER.

When using modulating control valves, it is necessary to specify either a non-zero fully open minor loss coefficient or discharge coefficient. This value is set in the property "Valve coefficient type".

While modulation is possible in any type of control valve, HAMMER SELECT series 4 only supports this behavior in PRV's.

Inaccurate results may occur if the valve becomes fully open or fully closed during a run or the pressure drops below vapor pressure at the valve. The percent closure for the valve can be found in temporary file C:\Users\FirstName.LastName\AppData\Local\Temp\Bentley\HAMMER\ PRVCLOSURE.TXT.

If the user selects False for "Modulate Valve during Transient", it is still possible to adjust valve opening during a transient run by changing the default value for "Operating Rule" from Fixed to an Operational (Transient Valve) pattern that the user has established under Patterns. In these patterns, the relative closure is a function of time. (See help topic Pattern Manager.)