17 replies to this topic

[process101]

Hi every one;
I have a simple question. How to find minimum suction pressure for centrifugal pump
when it is off? the equation i am using is
P=((head* specific gravity)/2.31))
Is this correct equation.
Thank you very much for help in advance.

[Art Montemayor]

The way to estimate the minimum suction pressure a centrifugal pump can tolerate is to guide yourself by the rated Net Postitive Suction Head required (NPSHr) for that specific pump operating at the specific capacity. The rule that I have followed all of these years is the following:

The Net Positive Suction Head Available (NPSHa) to the pump should be the highest value of either:

NPSHa = (NPSHr) (1.35)
or
NPSHa = (NPSHr) + 5 feet of fluid

There is no equation that I know of or have ever heard of. The equation that you cite certainly DOES NOT give you the "minimum suction pressure for centrifugal pump".

[fallah]

The way to estimate the minimum suction pressure a centrifugal pump can tolerate is to guide yourself by the rated Net Postitive Suction Head required (NPSHr) for that specific pump operating at the specific capacity. The rule that I have followed all of these years is the following:

The Net Positive Suction Head Available (NPSHa) to the pump should be the highest value of either:

NPSHr = (NPSHa) (1.35)
or
NPSHr = (NPSHa) + 5 feet of fluid

There is no equation that I know of or have ever heard of. The equation that you cite certainly DOES NOT give you the "minimum suction pressure for centrifugal pump".

I think in above two equations certainly Art meant NPSHr for NPSHa and vice versa.

[Art Montemayor]

Good eye, Fallah!

In my haste, I typed, copied & pasted the wrong "sub" symbol to the NPSH.
I've now corrected the equations by post editing. Thank you for watching out for my mistakes.

[djack77494]

How to find minimum suction pressure for centrifugal pump when it is off? the equation i am using is
P=((head* specific gravity)/2.31))

Is your inquiry about the pump's NPSH? I got a bit confused by you saying the pump is off. If you are asking about a simple static pressure calculation, then your equation is correct in Imperial Units of psi and feet. If you really do want NPSH, then follow the advice given by Art and fallah.

[JoeWong]

How to find minimum suction pressure for centrifugal pump when it is off? the equation i am using is
P=((head* specific gravity)/2.31))

Is your inquiry about the pump's NPSH? I got a bit confused by you saying the pump is off. If you are asking about a simple static pressure calculation, then your equation is correct in Imperial Units of psi and feet. If you really do want NPSH, then follow the advice given by Art and fallah.

Mr. Montemayor has advised you the minimum margin that you required for NPSHa over NPSHr when the pump is running. This is common and required. Of course, if you play around the equation, you would be able to derive the minimum suction pressure (e.g. when NPSHa = 1.35 NPSHr)

However, if you are just would like to know suction pressure when the pump is stopped (as Doug advised), i don't see why you need to know. Any reason in this case ?

[Zauberberg]

The question is not in place, since pump which is not running does not require any positive suction head - because there is no flow (and corresponding head loss) through the pump.

Even if you connect the pump directly to the vessel containing a liquid at its bubble point, there are no concerns since there is no flow. What are you looking for, exactly?

[process101]

Actually we need to find when the pressure at suction side becomes very low so the PLC system
can automatically shutdown the pump. the question is that @ what Minimum pressure it should shut down? My manager told me to find the pressure which is below the max. suction pressure and above the minimum suction pressure. the prob. is that i have the max. inlet suction pressure but in order to find the minimum suction pressure i used the equation which i posted in first query.
thank you very much for you help.

[Zauberberg]

Minimum suction pressure corresponds to the NPSHR converted to pressure, with fluid density at pumping conditions. Please also bear in mind that NPSHR is a function of flowrate: the pump will start to cavitate at higher suction pressure when it's pumping more fluid.

Also, if this pump is in critical service (feed or reflux pumps, for example) shutting the pump automatically might cause more problems then if you leave it to run for some time - and being aware by the alarm signal that you have cavitation problem going on. These issues should be thoroughly examined and evaluated, in order to achieve maximum plant safety and equipment protection, at the same time.

[process101]

Hi Mr. Zauberberg:
hope you are doing good. thank for your reply. You said that the pump will
start cavitate at higher suction pressure can you explain this please? Becouse i know that the pump cavitate when you have some vapor or gas in Liguid.
Thank allot

[Zauberberg]

Hello process,

Maybe my previous post was not written in a sufficiently clear manner. Look at any pump chart, and you'll see NPSHR vs Flowrate curve; as flowrate increases, the required NPSH also increases due to higher head losses in pump suction piping and impeller eye. It means you have to provide higher NPSH when you are pumping more fluid. Higher NPSH corresponds to higher suction pressure, for a given fluid density at pumping conditions.

In a simple words, if NPSHR = 3m when you pump 30 m3/h of fluid, in the case when you pump 45 m3/h of liquid the corresponding NPSHR will be higher than 3m. Take any pump chart you have - the graph is able to speak million words.

Best regards,

[djack77494]

For precisely the reason explained by Zauberberg, the use of a low suction pressure switch to trigger an alarm and/or pump shutdown tends to be a poor idea. Your suction pressure varies significantly depending on the flowrate through the pump, as well as the suction vessel's level and imposed pressure. A simple pressure switch intended to act based on one of these factors but not the others is unlikely to produce satisfactory results.

(On further reflection, an alarm might be OK, but an automatic pump shutdown is probably as likely to be hurtful as helpful.) Just my opinion.

[ankur2061]

Hi every one;
I have a simple question. How to find minimum suction pressure for centrifugal pump
when it is off? the equation i am using is
P=((head* specific gravity)/2.31))
Is this correct equation.
Thank you very much for help in advance.

Hi,

When I am doing pump hydraulic calculations initially with limited information I follow the following procedure for pumps taking suction from a pressure vessel:

Min Suction Pressure = Vessel Relief Valve Set Pressure + Static Head from Vessel Nozzle to center line of pump suction - Friction Loss in suction piping upto pump suction nozzle - Pressure drop for suction strainer

Points to note:
1. The relief valve set pressure eliminates the vagaries of trying to accurately determine the operating pressure envelope in the vessel and can also be used for determining the maximum suction pressure
2. For most general applications the line connecting the pump suction is from the vessel nozzle which is located at the bottom dished end (vertical cylindrical vessels) or the bottom end of a horiziontal vessel which is theoretically the lowest level with which the pump can be operated.
3. For most applications suction strainers are provided and as a first approximation a value of 1.5 psi delta P is fairly conservative.

Hope this helps.

Regards,
Ankur

[KVSS RAM]

Hi every one;
I have a simple question. How to find minimum suction pressure for centrifugal pump
when it is off? the equation i am using is
P=((head* specific gravity)/2.31))
Is this correct equation.
Thank you very much for help in advance.

The formula you mentioned is to convert the head in feet in to pressure.It is not applicable for arriving suction pressure.
The main objective is we should keep suction pressure above vapour pressure

The following formula will be helpful.

Mini suction pressure/ NPSH Available (Suction head Pumps) = Vessel pressure acting on the liquid surface + Static head-friction losses-vapour pressure

[ankur2061]

The formula you mentioned is to convert the head in feet in to pressure.It is not applicable for arriving suction pressure.
The main objective is we should keep suction pressure above vapour pressure

The following formula will be helpful.

Mini suction pressure/ NPSH Available (Suction head Pumps) = Vessel pressure acting on the liquid surface + Static head-friction losses-vapour pressure
[/quote]


The minimum suction pressure is not the NPSHa. For pump Hydraulic calculations I would recommend the min suction pressure to be calculated as I have mentioned in my post. The NPSHa would be the min suction pressure minus the vapor pressure at the maximum operating temperature.

As I have explained in my post, it is sometimes difficult to predict the vessel pressure acting on the surface of a liquid where there is a large operating pressure envelope. Hence the relief valve set pressure eliminates the guess work and can be safely used for suction pressure calculations. If the lowest pressure acting on the liquid surface can be accurately determined the same should be used for min suction pressure calculations.

Regards,
Ankur.

[process101]

Thank you very much for your reply:
I checked with my Manager another equation which is
Head =(Pressure/density*g(9.8m/s))
he said this is the correct equation.
As we have pump head from pump chart based on flow rate, we have density of the liquid.
so by using this equation tells you that how much min. pressure is required to drive the liquid up to
certain hight. Please put some comments on this.
Thank you all

[yomyom]

Hi every one;
I have a simple question. How to find minimum suction pressure for centrifugal pump
when it is off? the equation i am using is
P=((head* specific gravity)/2.31))
Is this correct equation.
Thank you very much for help in advance.

Please, what is the appropriate relationship between NPSHa, NPSHr and Pump cavitation?
Please help.

[Art Montemayor]

YOMYOM:

Please stop hi-jacking this thread and start your own thread. The subject being discussed in this thread is considered an important one and - like all other threads - should not be interrupted by outsiders with a different agenda or subject.

I will delete your post tomorrow, so please start your new thread quickly.