Pressure relief valve problem

[starman]

We ran the water tank dry today, refilled it and water ran through taps ok - but the pump kept running when they were shut until the PRV opened and started dumping it overboard.

It's a Jabsco Sensormax pump, the constant pressure type so no cold side accumulator, calorifier has a 3bar PRV and there's a hot side expansion vessel. That's on about 2bar.

Is there a logical way to fault find this or do I just keep replacing bits? The system ran fine before the tank went dry - I can't help think there's a link.

[bizzard]

We ran the water tank dry today, refilled it and water ran through taps ok - but the pump kept running when they were shut until the PRV opened and started dumping it overboard.

It's a Jabsco Sensormax pump, the constant pressure type so no cold side accumulator, calorifier has a 3bar PRV and there's a hot side expansion vessel. That's on about 2bar.

Is there a logical way to fault find this or do I just keep replacing bits? The system ran fine before the tank went dry - I can't help think there's a link.

Probably blown some limescale up to the PRV. Trying pulling out the PRV's red knob and twisting it about to clear the valve seat.

[ditchcrawler]

I would go for the pump pressure switch

[nicknorman]

Or a bit of limescale /dirt blocking the little orifice leading to the pressure sensor in the pump.

[MtB]

I would go for the pump pressure switch

 

 

I don't think it has one.

 

I'd say the PRV is passing because the pressure in the sytem is actually above 3 bar. A pump fault, but I don't actually know how these constant pressure pumps work.

 

 

Edit to add: No pressure switch according to the manufacturer. The motor speed is modulated to maintain constant pressure so there must be some sort of digital data sender.

Edited February 22, 2017 by Mike the Boilerman

[bizzard]

Ah, yes your both probably right. I don't know why but I read it as draining the calorifier instead of water tank.

[nicknorman]

I don't think it has one.

 

I'd say the PRV is passing because the pressure in the sytem is actually above 3 bar. A pump fault, but I don't actually know how these constant pressure pumps work.

 

 

Edit to add: No pressure switch according to the manufacturer. The motor speed is modulated to maintain constant pressure so there must be some sort of digital data sender.

Ok then, "pressure sender" rather than "pressure switch". But how does the pump know to switch off when all the taps are turned off.

 

As to how constant pressure pumps work, could be a swash plate pump whereby the are multiple pistons/cylinders and the stroke on the pistons is adjusted so as to regulate the output pressure. That's how the 175bar hydraulic pumps worked on our helicopters.

 

 

 

The bit on the left tilts (in response to pressure changes, not shown in this animation) so as to change the stroke on the rotating pumps.

 

Edited February 22, 2017 by nicknorman

[MtB]

I'd say the tilty bit in the OP's pump has jammed then.

[nicknorman]

I'd say the tilty bit in the OP's pump has jammed then.

 

Yes could be. Or at least isn't responding to pressure. Presumably there is a small piston/diaphragm being acted on one side by a spring and the other side by the pump's output pressure, linked to the tilty bit. If the output pressure is not finding its way to the piston/diaphragm due to a blockage then the pump will continue working flat out. It surely has to be linked to running out of water and disturbing the crud at the bottom of the tank.

[MtB]

 

Yes could be. Or at least isn't responding to pressure. Presumably there is a small piston/diaphragm being acted on one side by a spring and the other side by the pump's output pressure, linked to the tilty bit. If the output pressure is not finding its way to the piston/diaphragm due to a blockage then the pump will continue working flat out. It surely has to be linked to running out of water and disturbing the crud at the bottom of the tank.

 

 

Dunno about yours but my tanks never have crud in the bottom when I have occasion to see inside them. They are always pleasingly clean.

Reading the OP again it seems as thought the pump never stops running, so yes there must be a failure in the pressure feedback loop. The pump thinks the pressure is lower than it actually is.

[ditchcrawler]

Are you saying these pump motors run 24/7 and it just adjusts the flow?

[nicknorman]

 

 

Dunno about yours but my tanks never have crud in the bottom when I have occasion to see inside them. They are always pleasingly clean.

Reading the OP again it seems as thought the pump never stops running, so yes there must be a failure in the pressure feedback loop. The pump thinks the pressure is lower than it actually is.

 

We have an enclosed stainless steel tank tucked under the bow with a non-straight filler pipe. So no chance of seeing in it except with a boroscope. However lots of boats have water tanks comprised of the structure of the boat, and a loose fitting lid. Surely these must get some crud accumulating at the bottom as they get older and rustier?

Are you saying these pump motors run 24/7 and it just adjusts the flow?

 

No, they are not supposed to. Perhaps they have some flow sensing and turn on only when a flow is detected, with regulation of the output pressure by the sort of means I've mentioned. Well I suppose they must also have pressure sensing and turn the pump on when the pressure is low. So the logic would be:

 

Zero flow, pressure high = pump off

 

Zero flow, pressure low = pump on

 

Detected flow = pump on

[MtB]

I'd say when the electronics can't make the pressure fall below a defined (high) value even when the pump motor has been modulated down to a (pre-defined) speed, it shuts the pump motor off. Should dp/dt rise above a programmed value, the motor starts again.

[bizzard]

A Smart switch perhaps. I bought some fairy lights for Christmas at Range for £6. They're quite amazing. About 100 multi coloured LED's with 8 selectable different flashing sequences, auto timer, 6 hours on, 18 hours off. A wee micro processor I expect. I turned them on a few days before Christmas and they're still going strong, on 3 cheapo AA batteries!!! Made in China of course.

[system 4-50]

Deleted as Mikexx below has the answer. The sensormax can deliver up to 5 bar I was told.

Edited February 23, 2017 by system 4-50

[Mikexx]

We ran the water tank dry today, refilled it and water ran through taps ok - but the pump kept running when they were shut until the PRV opened and started dumping it overboard.

It's a Jabsco Sensormax pump, the constant pressure type so no cold side accumulator, calorifier has a 3bar PRV and there's a hot side expansion vessel. That's on about 2bar.

Is there a logical way to fault find this or do I just keep replacing bits? The system ran fine before the tank went dry - I can't help think there's a link.

 

While I accept this has been working previously, I think you should look at the instructions:

http://www.jabscoshop.com/files/USING%20A%20HIGH%20PRESSURE%20SENSOR-MAX%20doc550.pdf

 

"If your calorifier has a relief valve setting of 3.5 bar or less (virtually all European-built calorifiers), you will need a pressure-reducing valve, which will also act as a non-return valve."

 

It is possible that your pressure relief valve was stuck and is now working properly.

[Sea Dog]

Notwithstanding what Mike says above, do you have a strainer between your tank isolating valve and your pump? Many of the 'sticking' problems appearing after draining the tank will be down to debris from the bottom of the tank.

[starman]

Well it appears to be the pump. Swapped it for an old one I had and everything seems ok.

So that's a £250 pump lasting three years - rubbish, eh!

They don't make them anymore (wonder why) so I will have to plumb in an accumulator too.

I can't see anything obviously wrong with the old one - no debris etc - so I guess it's electronickery gone pop.

Thanks for all the advice.

[WotEver]

You could always use the old one in conjunction with a Square D pressure switch.

[MtB]

Well it appears to be the pump. Swapped it for an old one I had and everything seems ok.

So that's a £250 pump lasting three years - rubbish, eh!

They don't make them anymore (wonder why) so I will have to plumb in an accumulator too.

I can't see anything obviously wrong with the old one - no debris etc - so I guess it's electronickery gone pop.

Thanks for all the advice.

 

 

Yes they do. According to their website there is a new constant pressure model replacing this one, so you could get one of those.

[Tony Brooks]

From the little I hear about them they do not seem the most reliable of things. Those I have head operating seem to alter their speed so I suspect the motor is either like a stepper motor and driver or the mark space ratio of a pulsed supply is altered to maintain a given pressure.

 

I'm with the other Tony, ordinary pump plus D Square switch or the ordinary pump's pressure switch controlling a relay to switch the motor supply.

 

 

In my view too much "value engineering" - keep on reducing the components specs until you hit an acceptable level of unreliability/warrantee claims.

[starman]

10 hours ago, Mike the Boilerman said:

 

 

Yes they do. According to their website there is a new constant pressure model replacing this one, so you could get one of those.

It gets bad reviews - more complex, doesn't do the job as well and goes wrong a lot. Quite a feat!

[Phil Ambrose]

There is a lot to be said for the KISS principle also an element of " if it ain't broke don't fix it"

I have a strong suspicion that as with many things today we are getting too clever and less is more

Phil

[BruceinSanity]

10 hours ago, Phil Ambrose said:

There is a lot to be said for the KISS principle also an element of " if it ain't broke don't fix it"

I have a strong suspicion that as with many things today we are getting too clever and less is more

Phil

Something I often think about software and OS upgrades...

[starman]

On 23 February 2017 at 12:12, Tony Brooks said:

 

From the little I hear about them they do not seem the most reliable of things. Those I have head operating seem to alter their speed so I suspect the motor is either like a stepper motor and driver or the mark space ratio of a pulsed supply is altered to maintain a given pressure.

 

I'm with the other Tony, ordinary pump plus D Square switch or the ordinary pump's pressure switch controlling a relay to switch the motor supply.

 

 

In my view too much "value engineering" - keep on reducing the components specs until you hit an acceptable level of unreliability/warrantee claims.

So how would I fit a D Squared switch Tony? From what I can see the mechanicals of mine are the same as a standard one - the difference is at the nose where there is some sort of electrical pick up that links back into the motor and some sort of little nylon jobby (a technical term) that can move up and down. 

There's also a small sealed  box of electric bits called an 'EMI filter' on the 12v feed wiring to the pump.