A pressure drop dilemma – truth or fiction? Can anyone help solve the mystery?

The crime scene:

There is a water system containing all of the usual fittings being pumped around a building by a pump in the basement.
The pipework is DN150.
The water flowrate is around 18 litre/sec making the pipe velocity around 1 m/sec.
A gasketed plate and frame type heat exchanger (PHE), also in the basement, on the suction side of the pump with minimal pipework between the PHE and the pump.
The pipework is reduced down from DN150 to the DN65 connections on the PHE inlet and outlet.
The velocity through the connections is around 5.4 m/sec. An over-pressurisation unit that is also included in the system puts an over pressure of 1 bar into the water circuit.

What happens next:

Pump is switched on, the pump power reading checked and is found to be higher than stated on the pump curve.

The Investigation:

Check pressure drop readings across devices installed in the pipework, starting with PHE because it is closest to the pump and has the highest pressure drop in the system according to the data available on all of the other devices.
Pressure taken at PHE inlet.
Pressure taken at PHE outlet.
Pressure difference between the two readings is higher than indicated on the data sheet sent by the PHE supplier.
Mystery solved. The higher pressure drop across the PHE was not allowed for in the pump sizing and so the pump is drawing more power than it should be.

Case Closed. Or is it…?

Maybe the real culprit escaped unscathed? It is hard to blame anything other than the PHE (when the measured difference of the pressure readings taken across the inlet and the outlet of the PHE is so damning), but could it actually be the pump that is at fault?

Possible alternative explanation is as follows – true or false?

The pump is slightly undersized for the duty being asked from it. This could be due to the pump curve not being accurate or because the motor horsepower is not as stated “on the tin”. A struggling centrifugal pump tends to cavitate. However, the over pressure of 1 bar added to the system by the over pressurisation unit is suppresses the air from separating out of the water and so the cavitation at the pump impellor is also suppressed. This allows the pump impeller to maintain a certain amount of performance.

The PHE on the pump suction side has the highest pressure drop (even before the values were taken in the investigation), and so is putting up the greatest resistance to the flow. The impeller churning away is sucking the water and a slight negative pressure builds up at the PHE outlet. To maintain a balance, if you have a negative pressure on the suction side of the pump, then a slight positive pressure builds up on the PHE inlet. Therefore, any pressure differential readings taken between the PHE inlet and the PHE outlet are exaggerated and give the impression that the pressure loss across the PHE is greater than claimed on the manufacturers data sheet.

Conclusion

An over pressure put into any water system changes the dynamics of that system and effectively adds a “head” pressure onto the pump that has to be allowed for when selecting a suitable pump motor power. The over pressure suppresses the cavitation and if the motor is not strong enough then a negative pressure builds up on the suction side and a plus pressure occurs on the flow side of the pump.

A bit of maths

The system loss is calculated to add 2.5 metre head onto the pump.
The pressure drop across the PHE is equivalent to a 7 metre head.
Total head: 9.5 metre with a flow of 18 litre/sec
Over pressure added by pressurisation unit = 10 metre

Motor horsepower required with no over pressure added = (18 x 9.5) / 95 = 1.8 horsepower
(95 is a constant to calculate horsepower)
Motor horsepower required with over pressure added:
Total loss = (OP + SL) x SqRoot(OP + SL) all divided by SqRoot(SL)

Where OP = over pressure
SL = system loss
SqRoot = square root

Adding in the figures for SL=9.5 and OP=10
Result = 27.9 metres which the motor power should be based upon and not the original 9.5 metres (this makes the motor power required nearer to 6 HP as opposed to 2 HP derived from no over pressure being added)

The question is:
Does the over pressure added to the system hide an undersized pump motor, or was the PHE pressure reading given by the manufacturer too low/inaccurate?

Getting the most from a Heat Pump Heat Exchanger