Where's my flowrate?

A client was baffled to find the flowrate he was getting from his Sandpiper model HDB4 air operated double diaphrahm pump was less than 30% of the expected flow at the specified head of 1.5 bars. A site investigation revealed the cause and provided a useful lesson.

Problem

The client complained about the low flow rate he was getting from a Sandpiper model HDB4 air operated double diaphragm pump on a fine slurry. The pump was delivering less than 30% of the expected flow at the specified head of 1.5 bars. The client was using 465 metres of 200 m PVC pipe.

Investigation

After a site inspection, it was determined that consideration had not been given to the impact of acceleration losses in the very long discharge line

The flow in the suction and discharge piping of a reciprocating pump is not continuous. The pump must accelerate and decelerate the liquid in the suction and discharge pipe with each cycle.

The acceleration head for a reciprocating pump is calculated as follows:

h=LVNC/gK

h = Acceleration head (in metres)
L= Length of the pipe (in metres)
V= Average liquid velocity in the pipe, (in metres/ sec)
N=Speed of the crankshaft (revolution/minute) or strokes per minute
C= constant depending on pump type. For a double diaphragm pump it is 2.0
G= Gravitational constant (9.8 m/s2)
K= constant depending on fluid compressibility (for non-compressible liquid like water K=1.4)

The value of C

  • 1 for simplex pumps (single piston, plunger, or maybe reciprocating oil to move a diaphragm.)
  • 2 for duplex (two displacers)
  • 4 for triplex (three displacers )
  • 6.5 for quadruplex (four displacers)
  • 9 for quintuplex (five displacers)

Calculations showed the following acceleration losses

  • At a flow of 700 l/min, acceleration losses were 245 metres (total losses were 260 metres)
  • At a flow of 300 l/min, acceleration losses were 45 metres (total losses were 60 metres)

Solution

To solve the above problem, a pulsation dampener was installed on the discharge of the pump. The pulsation dampener was a BLACOH SENTRY Pulsation Dampener placed at the pump's discharge and it produced a near steady fluid flow up to 99% pulsation and vibration free, eliminating the impact of acceleration head. So the Sandpiper HDB4 now produced the required flow.

Some minor modifications were made to pipework.

Two separate suction lines were installed because with one common suction line, both pumps could not be run together. If they are, one pump will be dominant and the other will not prime. Additionally, the Y-type connection needed to be utilized to manifold the two discharge lines.

The balls in the pump should work as a non-return valve, but in case of one ball being stuck open, the liquid will get back to the suction tank through the standby pump. The non-return valve will work as an extra safety device. The non-return valve should be placed after the dampener, so it is not exposed to pulsation which would cause excessive wear.

We also advise to use an isolation valve, as per the drawing, for maintenance purposes.

Result

After the client changed all his pipe work and installed a dampener on each pump, the required flowrate was achieved.

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