Diaphragm pump supplier won't give NPSHR, only maximum suction lift

[craftychemist]

Hello,


Most of the time I do work on centrifugal pumps, but this time I need to design a diaphragm pump.

I've conducted calculations on a Husky 2150 double diaphragm pump to have an NPSHA of 6.3m including acceleration head (given it's a reciprocating pump), but the pump supplier doesn't provide an NPSHR curve, only says that the Maximum suction lift = 6.1m.

The maximum suction lift can be expressed as Suction lift = P(b) - (Ls + Vp + hf + NPSHR), where:
Pb is Atmospheric pressure, = 10.3m head
Ls = elevation difference of water level to pump intake nozzle, = ?
Vp = vapour pressure of water at 20 deg C,= 0.24m head
hf = friction losses in pipe = ?
NPSHR is usually from the pump curve.
(Note, the above is based on this formula: https://www.mechequip.com/a-simple-way-to-determine-suction-lift/).

Hence, I assume with a little rearranging of the equation, I should be able to derive NPSHR, (Supposedly diaphragm pumps have a flat NPSHR curve, so I assume that the NPSHR would be the same for the conditions during maximum suction lift?) however, I don't know what conditions they used to empirically derive maximum suction lift so I don't know their hf, or Ls. Is there a standard experimental setup for this so I can make some assumptions?

What is normally the way of checking if a double diaphragm pump is suitable given most suppliers only provide maximum suction lift? They said that diaphragm pumps don't normally have NPSHR curves because they're less vulnerable than centrifugal pumps to damage from cavitation, but articles I've read do talk about the applicability of checking the NPSHR.

I've attached the calculation for reference.

 

[LittleInch]

If you've got 6.3m lift capacity, that's about 4m NPSHR plus your friction losses and a bit of acceleration head.

I don't know how they do it, but my guess is much the same as they do it for centrifugal pumps by throttling the inlet pipe and measuring absolute pressure at the nozzle.

As you say, diaphragm pumps are not really susceptible to cavitation damage so they probably don't really care about it, it's the lift performance most people wan to know about. There is though a practical limit where the diaphragm will flex too much and fail to work properly which is probably where the maximum lift figure comes from.

so at 6.3m NPSHA I would say you're fine as I assume this is calculated at the inlet nozzle?

 

[pierreick]

Hi,
let you study the document attached together with the link,

https://pumpengineers.co.nz/wp-content/uploads/2023/03/wilden-pump-users-guide-ii.pdf

relation between Suction lift and NPSHR is unclear to me, are you sure it's not the NPSHA?
see link below:

Calculate NPSHa for a Suction Lift Condition

This is the second article in a five-part series. Read the first part of the series here. Last month, we worked through an example for a simple system calculation of net positive suction head available (NPSHa) based on a 10-foot flooded suction with 68 F water at sea level. The head due to...

www.pumpsandsystems.com

Pierre