The information that follows is based on an application where the potential for damaging surge conditions within a Sewage Pumping Station was determined by a surge analysis report:
Pump Non-Return Valves
The analysis has shown the pump non-return valves must be capable of matching a rate of flow decay of 2.45 m/s (based on nominal 400mm i.d. Ductile Iron pipework) to zero in 0.21 seconds. This equates to a flow deceleration of 11.60 m/s
With a typical flow velocity of 2.00 m/s, i.e. a deceleration rate of 1.33m/s*2, a standard swing check valve is normally capable of closure in approximately 1.5 seconds from a fully open position. As a result of the analysis a rapid acting spring assisted type check valve was recommended on all pumps.
Calculations
Initial flow rate of 2.45 m/s (8.04 fps) will decelerate to zero in 0.21 seconds
Deceleration of 11.6 m/s*2 (38.1 ft/s*2).
According to the dynamic characteristic curves below, the Surgebuster should only allow about 0.6 fps of reverse flow before closing. Projecting the curve out for the standard Swingflex indicates that it would allow about 2.6 fps of reverse flow before closing.
Please Note – actual application data for the above example is considered confidential. For further information and an introduction to the relevant water authority please contact MGA Controls Limited referring to the above Application Reference.
Time to Closure and Surge Pressure Calculations
The SwingFlex swing check valve
With reference to the above table:
Flow Deceleration at 38 fps*2 Ξ 2.6 fps (high into ‘severe slam’)
Time to Closure:
Reverse Velocity at Time of Closure / Deceleration + Initial Time of Flow Decay = Time to Closure
2.6 fps / 38 fps*2 + 0.21s = 0.28 s
From the Joukowski equation, Pressure Surge:
h= av/g
a – pipe wave velocity
v – reverse velocity
g – gravity 9.8m/s*2 (32.2 fps*2)
h = 3714 x 2.6/32.2 = 299.9
299.9 x 0.4335 = 130.0 psi = 9.0 Bar*
The Surgebuster swing check valve
With reference to the above table:
Flow Deceleration at 38 fps*2 Ξ 0.6 fps (marginally into ‘mild slam’)
Time to Closure:
0.6 fps / 38 fps*2 + 0.21s = 0.23 s
Pressure Surge:
h = 3714 x 0.6/32.2 = 69.2ft
69.2 x 0.4335 = 30 psi = 2.1 Bar*
* Note – The surge pressure is addition to the static line pressure.
References:
Val-Matic Drawing No.: SS-1886, Dynamic Characteristics of Various Check Valves
Val-Matic White Paper, Surge Control in Pumping Systems
The above calculations are based on independent test data and are subject to verification by Val-Matic
Please Note – actual application data for the above example is considered confidential. For further information and an introduction to the relevant water authority please contact MGA Controls Limited referring to the above Application Reference.