pump suction gauge is ZERO,DISCHARGE 40 PSIG.

[moideen]

As I visit the air cooled chilled water plant, found the chilled water pump suction gauge is zero; the discharge pressure is 40psig.as far as I know, if there is no sufficient suction will cause the cavitations. But I was told there is not much any problem on this system.

THANKS

 

[Artisi]

Is it a compound gauge, pressure or vacuum gauge?
If it is reading zero, either fit a new gauge or have the existing gauge checked.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

It is not usual for the inlet guage for such a system to read below 0 psig (i.e. below local atmospheric pressure). This might not damage the pump as the pump can often work well at 4-5m NPSH (or what you might think of as -7 psig)

However operating at this pressure might draw air into the system and if the water become hot might create vapour bubbles.

There are a few explanations:
1) Your system works fine at pressures lower than 0 psig, but you guage is only good for positive pressures. As artisi says, fit a guage which can read pressures below atmospheric.
2) Your guage is isolated or broken
3) Your pressurizing system isn't working and you need to add water to pump it back to what it should be
4) Your expansion tank is broken or not pressurized correctly

Learn more about how your system is designed to work and then you will be able to work out if it's not working correctly.

PS Please don't use CAPITALS in your post - this is seen as SHOUTING and is not allowed.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

Net Positive Suction Head is measured in absolute units therefore add 14.7 psi to your PSIG reading.

First rule of maintenance: if it ain't broke, don't fix it.

 

[moideen]

littleInch: this is new gauge, and there have two gauges for two pumps. It shows same pressure. There is pressurization unit, but I don’t know its condition. anyway I will recheck it. Thank you lean me the problem of “capital letter.” sorry English is not my native.

 

[LittleInch]

You need to find the normal operating point in the operating manaual and then see if your system is operating correctly or not.

I suspect your system needs a bit of "pumping up", but how much you need to work out from the operating manual.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

I now assume that the inlet gauge is a pressure gauge, is this correct?
If it is a pressure gauge, then if the inlet is at negative pressure the gauge will read zero. You need to fit a compound or a vacuum gauge to establish what condition the inlet to the pump is operating under.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[moideen]

 

[moideen]

discharge gauge

 

[moideen]

artisi : your advise is valid, i will fix compound gauge.

 

[itdepends]

That fitting underneath the suction gauge (the one reading zero) isn't an isolating valve for the gauge is it? It doesn't look like a normal reducer and could be a valve with a handle missing (spindle on the far side from where the photo was taken).

I can see one in the background on a different gauge that is obviously a valve. Assume if that's what it is- that the valve is open.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[mbolamr]

Hi moideen,

This is a quite interesting observation and denotes your knowledge on theory.

1. We should note that some specific pumps can work with very low NPSHReq, most of the fire pumps fall within this category (Please have a look at NFPA Standards). Your pump seems to be within those with capability of very low NPSHReq.

2. Assuming that your pressure gauge is good, we have to find the reason of why the equipment is operating properly. One explanation might be because the NPSHAvailable is higher than the NPSHReq. So, please, check the NPSHAv, the pump might be sucking from one tank, read the pressure reading at the tank in this case. By calculating the NPSHAv using the standard formula that we are used to apply, you can calculate it easily. Check your P&ID to have a more accurate calculation result. Then, go to the pump specification, and have from the supplier the NPSHReq by this specific pump. If al is OK I mean NPSHAv > NPSHReq + 20%, you can go to the next step.

3. As BigInch said NPSH is an absolute pressure reading PSI(a) while your manometer reading as it can be seen from the photo is a gauge reading PSI(g). That means if from your calculation, you find a NPSHa of about 5m, then at the gauge you should have

PSI(g) = PSI(a)- ATM
PSI(g) = 5 - 10 m
PSI(g) = -5m

And here is the secret: In this specific case, it will be more interesting to have a compound pressure gauge which can read negative value of pressure (or if you want, vacuum) instead of simple pressure gauge which cannot read negative values. And you wil see that your needle will move in the CounterClockWise direction!

This will be helpful troubleshooting the system in the future in case there will be some issue.

Hope this help.

Good Lick.

Martial.