PRV blow-off

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julianmingham

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Hi all

I've got a persistent PRV blow-off issue that I'd like to get forumites input on.

Here's the background:

Boat is a Westerly Oceanlord
Pressurised fresh water pump is a Flojet R4405-143 rated at a flow of 12.5 LPM and a cut-out pressure of 35 PSI
There is an expansion tank fitted, CW269 from Cleghorn Waring
The PRV on the calorifier is rated at 3 bar (which is 43.5 PSI)
I am using a Halfords digital tyre pressure gauge to measure the air pressure at the expansion tank

I've been suffering from a PRV blow-off issue. The blow-off from the PRV is led to a plastic bottle in the bilge, and when the water in the calorifier was hot it would fill a 2 litre bottle in less than 24 hours. When the water in the calorifier is cold I do get some blow-off but at a much lower rate

I've so far taken the following steps:
1. I turned the knob on the PRV to make sure that it seats well, but that made no difference.
2. I replaced the PRV on the assumption that it was faulty. That did make a difference, i.e. the discharge into the bilge was reduced, but it was still discharging albeit at a slightly lower rate.
3. I replaced the expansion tank, as further investigation discovered that the tank was not maintaining air pressure. I set the air pressure according to the instructions from Cleghorn Waring, i.e. 2 psi below the cut-out pressure of the tank (so I set it to 33psi). Sadly this made no difference to the discharge into the bilge.
4. I measured the air pressure at the expansion tank with the fresh water pump switched on and the water in the calorifier cold. I was surprised to find that it measured 41psi.
5. I switched the pump off, ran the tap to release the pressure, and released all of the air from the expansion tank.
6. I then pumped the expansion tank back up to 33psi
6. I switched the pump back on and measured the air pressure at the expansion tank - it was 42psi

I'm now wondering what to do next, ...
I'm assuming that the measurement at the expansion tank with the pump switched on is actually the cut-out pressure of the pump ? If that is the case then it shouldn't be 42psi.
I'm starting to think that I need to change the fresh water pump, but not sure whether to change it for another one the same spec. or to go for a lower pressure in order to increase the difference between the cut-out pressure of the pump and the pressure that the PRV will start to lift at ?

Can someone advise please ? Thanks
 
Where is the expansion tank? I have two. One is on the cold water supply line just after the pump, the orher is in the hot water supply line just after it leaves the calorifier.
 
I'd start by fitting a 4bar PRV, assuming your calorifier can withstand 4bar - which it should. Many calorifiers now come with 4bar PRVs as standard. Much cheaper and easier than changing the pump.
 
julianmingham said:
Hi all

I've got a persistent PRV blow-off issue that I'd like to get forumites input on.........................
Click to expand...

I agree with you that the most likely cause of the problem is that the pump is not cutting out at the correct pressure. I guess the pressure switch is not adjustable but it is worth investigating .
Even if not adjustable it is available from Jabscoshop http://www.jabscoshop.com/flojet-ma...sure-controlled-pump-with-internal-bypass.htm as a spare part. Expensive but not as expensive as a whole new pump.

Alternatively do as pvb suggests and fit a higher rated prv ..... if that is ok for the rest of the system.

However it would be a good idea to post a diagram of your system to be sure that we understand it correctly.
 
Last edited:
Just looked at the manual for the pump. It states quite clearly that it should not be used with an accumulator tank, so that may be part of the problem. On most of these type of pumps, the pressure valve has some adjustment: there's a small screw in the centre of the pressure switch which you can back off slightly to drop the pressure. Not sure if there's one on your pump, as the manual doesn't mention it but the drawings show what could be a screw cover in the centre of the switch housing which could be covering an adjuster. Worth having a look before starting to replace PRVs on the calorifier.
 
pvb said:
Read step 3 in the OP!
Click to expand...

Fair comment but reading the tank instructions it would appear that the internal pressure should be set to 2 - 3psi below the pump cut-in pressure whereas the OP seems to have set the tank pressure to 2 - 3psi below the pump cut-out pressure which he shows as 35psi.
 
Thanks for replies so far

I may not be able to provide a full diagram (I'm not on the boat and anyway some of the pipework is impossible to see/feel). From the pump the pipework goes to what I believe is a non-return valve and from there directly to a T piece at the bottom of the expansion tank. Does this sound correct or should the expansion tank be on the output side of the calorifier ?

To the point about the pressure it should be set to, ... this additional tank can be used either as a accumulator or as an expansion tank. I'm sure someone will correct me if I'm wrong, but my understanding is that if it's on the cold side it's an accumulator but if it's after the non-return valve it's an expansion tank. Furthermore, that the pressure in an accumulator should be just below the cut-in pressure of the pump and that the pressure in an expansion tank should be just below the cut-out pressure of the pump ?
 
Julian, I am going from the instructions from the Cleghorn Waring web site for the vessel you have (CR269) http://www.cleghorn.co.uk/files/Accumulator%20and%20Expansion%20Tank%20Instructions%20ZPWL4%20doc595.pdf. Do you have pressurized cold water as well as hot water? If so then the NRV should be on the feed to the calorifier after the take-off for the cold taps. When you say "this additional tank" is there another tank elsewhere?

Has this problem recently started or has it always been evident?

You need to have an idea of the size of the calorifier to calculate the correct size of tank required.
 
thanks for responses

Yes I have pressurised cold as well as hot water, and indeed the NRV is on the feed to the calorifier after the take-off for the cold taps. I note that the information in the link above , i.e. http://www.cleghorn.co.uk/files/Accumulator and Expansion Tank Instructions ZPWL4 doc595.pdf
states that an expansion tank should be set at the same pressure as the cut-out pressure of the pump, whereas the Cleghorn Waring literature supplied with the CW269 tank said that the pressure should 2 psi below the cut-out pressure of the pump for an expansion tank

My reference to the additional tank I meant the CW269.
I've only had the boat for 6 months so I don't know the full history, but I suspect that this has been a problem on this boat for years.

The calorifier is 30 litres, so the expansion tank may be under-sized.

So I think my options are:
a) fit a larger expansion tank
b) fit a PRV with a higher rating, e.g. 4 bar
c) reduce the pressure from the pump (which I may be able to achieve by changing the pressure switch on the pump)
 
Lots of systems don't have expansion tanks, and work fine. I reckon your main problem is that the pump is generating a pressure very close to the PRV's release pressure. The cheapest solution is a higher-rated PRV.
 
Or route the PRV exit though a non return valve to a skin fitting on the stern :-)

pvb said:
Lots of systems don't have expansion tanks, and work fine. I reckon your main problem is that the pump is generating a pressure very close to the PRV's release pressure. The cheapest solution is a higher-rated PRV.
Click to expand...
 
julianmingham said:
So I think my options are:
a) fit a larger expansion tank
b) fit a PRV with a higher rating, e.g. 4 bar
c) reduce the pressure from the pump (which I may be able to achieve by changing the pressure switch on the pump)
Click to expand...

It may be that someone has already meddled with the pump cut out pressure adjustment and increased it although the instructions say it should not be altered,

If it gets the pressure up to 41 or 42 psi it cannot be cutting out at 35 psi surely. I'd look into this as a possibility before changing the PRV.
 
Thanks all, I'll look at changing the pressure switch on the pump.
I don't want to route the PRV exit somewhere else and risk loss of fresh water - that may be OK for short coastal trips but for longer trips I wouldn't want to be losing precious fresh water
 
I would try setting the accumulator pressure at around half that suggested. As the water fills and the bladder reduces the air space the pressure will soon increase.

I would suggest 1 Bar / 15 psi. 33 psi seems way too high.
 
pvb said:
Lots of systems don't have expansion tanks, and work fine. I reckon your main problem is that the pump is generating a pressure very close to the PRV's release pressure. The cheapest solution is a higher-rated PRV.
Click to expand...

Only when they are cold water only, hot water has to expand and fitting a heavier PRV will not stop the problem, only blow the system elsewhere!
Forget all the pumps and pressures nonsense, your system is set up wrongly.
 
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