Pump troubleshooting 3

[miya2014]

Does anyone know the physical difference between an absolute pressure gauge and regular pressure gauges. I am not looking for definitions or differences between absolute and gauge pressure, I am looking to physically identify an absolute gauge. I want to order a few but I am not sure. I intend to install these gauges at the suction of a pump to help monitor hydraulic conditions. I am not able to tell if the attached gauge can read absolute or not

 

[JJPellin]

The pictures in the attachment show the gauges sitting on the bench, not connected to anything and reading 0.00 psi. That would tell me that it is reading in psig and not psia. I have never seen a gauge like this that reads in units of psia.



Johnny Pellin

 

[miya2014]

what about this one, this seems to be an absolute gauge,

 

[Compositepro]

Neither of those gauges read absolute pressure. Absolute pressure gauges require a reference vacuum inside the gauge. Literature for these gauges will explicitly state that they read in absolute pressure (psia not just psi). However, barometric pressure rarely varies by more that one psi unless you are at high altitude. So is it very important on a gauge that reads to 100 psi?
I, too, have had difficulty finding absolute pressure gauges in the past. In the lower ranges they are sometimes called manometers instead.

 

[bimr]

The absolute pressure gauge will say "Absolute Pressure".

Here is a link to one.

http://www.wika.us/upload/DS_HP_1500_8_5_Absolute_en_us_17141.pdf

 

[miya2014]

Thanks gentlemen, very useful contributions. have any of you found these to be useful at the suction of a pump for monitoring purposes. see my application in the picture. I am thinking of installing absolute gauges at the suction of the pump, one upstream and one downstream of the strainer (filter). the mechanical seal has failed 4 times in 3 months, product is sodium sulphide (most times very very dirty from the bottom of pits in refinery). the product comes in tank trucks and is offloaded into 150,000 storage tanks from the truck rack in the picture. I just wish to monitor the suction and discharge pressures to better understand whats going on

 

[LittleInch]

Wouldn't it just be easier to install a differential pressure guage?

Filters like this in the inlet line are always bad news, especially if you have virtually no positive head.

I suspect what you really need here is a pump capable of handling your product as it is without needing a strainer and then placing one downstream of the pump.

I'm not familiar with sodium sulphide, but if it is "dirty" then the rather small filter you have is going to get clogged up pretty fast.

Time for a re-think.

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

 

[lilliput1]

Instead of using multiple gauges instead manifold together to one pressure gauge the (3) pressure pickup points with branch valve each. Open the branch valve of the pressure point to be measured and close the other two. Also make sure the strainer has a blow off valve so the strainer does not have to be removed for cleaning.

 

[lilliput1]

Also if the absolute pressure gauge is too expensive consider using a compound gauge instead. This would have + gauge pressure readings in psig and also - in Hg vacuum. You do have to convert readings to absolute pressure though.

 

[miya2014]

thanks LittleInch,
Wouldn't it just be easier to install a differential pressure gauge? -------------It would, my hope is to use the gauge to monitor the pump suction, not only the differential pressure across the strainer. so the gauge downstream of the strainer would double up as the pump intake gauge.
I'm not familiar with sodium sulphide, but if it is "dirty" then the rather small filter you have is going to get clogged up pretty fast-----------yes it does get clogged up pretty quickly, we try to keep up with cleaning the strainer basket every day.
I suspect what you really need here is a pump capable of handling your product as it is without needing a strainer and then placing one downstream of the pump ------------ you are absolutely right, but for now I am trying to make this work for as long as possible. during project development, I did not get a very accurate definition of the product, so this pump is really not suitable for this product, I assumed it would be clean sodium sulphide. At this point, I intend to let ops continuer using the system as is while I gather hydraulic data, basically buying some time to come up with a more permanent solution.

 

[miya2014]

Lilliput1 thank you "Also if the absolute pressure gauge is too expensive consider using a compound gauge instead" --------- is this identical to a vacuum gauge ? I currently have a vacuum gauge at suction that reads -Hg vacuum and +psig. Its on the thermal relief piping on picture attached above.

 

[LittleInch]

Fair enough - you could think about maybe using a duplex filter which should make it easier to remove the crud on a more frequent basis without having to stop operation. something like this?

I think they make them a similar or same size between flanges as a single one so oyu can just replace the undersized thing you have with this.

Some you can run both together at the same time to reduce your DP.

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

 

[miya2014]

LittleInch thank you, I will definitely look into this duplex filter, hopefully I have enough room to fit it in.

 

[lilliput1]

Google compound gauge and view image. Select appropriate range for your service. Note manifolding the pressure pickup points to one gauge improves accuracy and reduces cost.

 

[danw2]

I'm clueless on the filter stuff but I can tell you you're making a mistake to consider an absolute pressure gauge. I can assure you that no one, besides yourself, will understand why an absolute pressure gauge reads almost 15psi when there's "no pressure". It will be broken from the day it is installed until the day someone breaks it trying to zero it.

The times we've sold one I'm the guy who gets to (try to) explain why it isn't broken. I've heard heated statements that it has to be broken because it shows 14-15psi at atmospheric pressue. "There ain't no pressure there." (verbatim). If they're lucky, I get there before they've tried to zero the pointer. If not, it's probably toast because the last guy who knew what a pointer puller was retired in 1989.

Be my guest, go ahead and try explaining and they'll look at you like you're from Mars and tell you don't know one part of your anatomy from another (in colloquial terms). You're telling someone who knows darn well that when that noisy vacuum pump runs that it creates a vacuum but that new fangled gauge is reading positive? Uh-huh. Who's the crazy one? "Who you callin' a liar, boy?" Go ahead use that college talk, "measures with respect to . . ." That'll impress 'em. Been there, done that too many times (my company sells pressure gauges and transmitters).

Given the relatively minor effect that barometric pressure changes have on what you're trying to read, I'd go with the previous recommendation (lilliput1) for a gage pressure compound pressure gauge that displays vacuum and positive pressure on the same scale, vacuum as negative (minus to the shop boys) and positive pressure as positive.

I've found that operators do tend to understand compound gauges, especially if they've ever messed with automobiles and intake manifold pressure. Vacuum is minus. Positive pressure positive. "Now you're talking sense, pal."

If you need an absolute value, use a compound or vacuum gauge and then look up the barometric pressure for your locale on the web and make a (relatively minor) correction to the gage pressure reading.

2) I, too, don't know what sodium sulphide is, but if it eats 316 stainless you'll need a diaphragm seal to isolate and protect the gauge internals from the process. This is critically important, as benign as sodium sulphide might be, you don't want it coming out the gauge case because the bourdon tube failed and leaks.

If the medium is sticky or tends to solidify the medium could clog up the socket or the bourdon tube. Another reason that an isolating diaphragm seal might be needed.

If a diaphragm seal is needed, let the vendor do it. It takes a 'hard' vacuum pump and some skill to backfill the system and seal it. It's not something the ordinary shop is equipped to handle.

Diaphragm seals are 'filled systems' and either Mr. Boyle or Mr. Charles (can't remember which) informs us that a pressure measuring filled system will also act like a thermometer, meaning process temperature will affect the pressure readings. Fact of life.

Same caveats apply to differential measurements. And while I'm pontificating, taking two gage pressure measurements and subtracting them is only an approximation of differential pressure (DP), when one considers the accuracy of most people's pressure gauges and the skill needed to read two locations at the same time.

3) Claimer: Please, please buy a $100 pressure gauge from my company at $22 gross margin so I can retire tomorrow on my $1.50 commission.

Dan

 

[danw2]

This article (I'm surprised it's still at its URL from several years ago) has excellent advice on test pressure gauges, particularly on a snubber/isolator, and the author doesn't even have a vested interest of $1.50 at stake.