Pressure release valves

[blackrose]

What pressure are most PRVs on calorifiers set to open at? This is obviously supposed to be higher than the water pressure but by how much?

 

I can't see any water coming out of my PRV but there's definately a dribble because there's a streak of wet blacking below the skin fittng - even when the calorifier is cold. I've tried twisting the knob on the valve to reseat it but it doesn't dry out the wet patch I can see outside.

[wonderdust]

I think it is around 3 bar which is about 43 psi if thats any help.

 

Found this if it any help about 3/4 of thee page down says it is normal to leak when the water is hot!

click

Edited March 21, 2008 by wonderdust

[chris w]

What pressure are most PRVs on calorifiers set to open at? This is obviously supposed to be higher than the water pressure but by how much?

 

I can't see any water coming out of my PRV but there's definately a dribble because there's a streak of wet blacking below the skin fittng - even when the calorifier is cold. I've tried twisting the knob on the valve to reseat it but it doesn't dry out the wet patch I can see outside.

 

 

This comes up with regular monotony. When water is heated, it wil expand by around 3-4% depending on final temperature. This expansion has to be accomodated or else the calorifier will burst because liquids are incompressible.

 

The PRV is forced open when the water is hot and a dribble will occur all the time it is hot. To prevent this happening, and thus to prevent losing the hot water you've heated up, one can fit an expansion vessel to the calorifier (tee'd off for example from the hot water outlet at the top of the calorifier). This is a small cylinder (around 8-10 litre total capacity) comprising half air and half water separated by a rubber diaphragm which stretches as the pressure builds up to accomodate the water expansion.

 

The expansion vessel's pressure (set with a bicycle pump via a valve) should be set a few psi above the pump's cut-out pressure (typically 30psi) but below the PRV release pressure, the latter pressure being around 3 bar (42psi) typically. So an expansion vessel's pressure would be around 35-38psi typically.

 

A PRV can also dribble if calcium or other crud builds up under its seal. This can usually be cleared by twisting the PRV release knob a few times. This is the issue if the PRV is leaking when the water is COLD. For the reasons articulated above, the PRV will ALWAYS dribble when it is HOT unless an expansion vessel is fitted. The "leak" should be directed to the outside of the boat via a skin fitting although it is not unknown for some daft builders to let it dribble into the bottom of the boat!

 

Chris

Edited March 21, 2008 by chris w

[blackrose]

This comes up with regular monotony. When water is heated, it wil expand by around 3-4% depending on final temperature. This expansion has to be accomodated or else the calorifier will burst because liquids are incompressible.

 

The PRV is forced open when the water is hot and a dribble will occur all the time it is hot. To prevent this happening, and thus to prevent losing the hot water you've heated up, one can fit an expansion vessel to the calorifier (tee'd off for example from the hot water outlet at the top of the calorifier). This is a small cylinder (around 8-10 litre total capacity) comprising half air and half water separated by a rubber diaphragm which stretches as the pressure builds up to accomodate the water expansion.

 

The expansion vessel's pressure (set with a bicycle pump via a valve) should be set a few psi above the PRV release pressure, the latter pressure being around 2.5 - 3 bar typically. So an expansion vessel's pressure would be around 45psi typically.

 

A PRV can also dribble if calcium or other crud builds up under its seal. This can usually be cleared by twisting the PRV release knob a few times. This is the issue if the PRV is leaking when the water is COLD. For the reasons articulated above, the PRV will ALWAYS dribble when it is HOT unless an expansion vessel is fitted. The "leak" should be directed to the outside of the boat via a skin fitting although it is not unknown for some daft builders to let it dribble into the bottom of the boat!

 

Chris

 

I've got an expansion vessel. Just given the knob a few more twists to try and clear the crud but if this doesn't stop the dribble do I need a new PRV?

Edited March 21, 2008 by blackrose

[chris w]

Hi Mike - I hadn't spotted it was you who posed the original question. If it's leaking when cold, then either it's crud or the PRV has a fault. They are pretty inexpensive items actually so if it persists you may want to swap it out. (eg: Screwfix)

 

Chris

[steelaway]

I've got an expansion vessel. Just given the knob a few more twists to try and clear the crud but if this doesn't stop the dribble do I need a new PRV?

 

 

My original PRV had vent holes above the diaphram and was dripping through a pin hole in the diaphram and out through the holes. I replaced it with one with out vent holes. Although it can still fault through grit ect on the valve seat it cant now leak out the top of the diaphram. My advise is to try and obtain the type without vents.

 

Alex

[blackrose]

I'll see what happens to the PRV after I fit my new waterpump this weekend. The pump's leaking too!

 

Am I right in thinking that as long as I switch off the pump and release the pressure by opening a hot tap, I can remove the PRV from a vertical calorifier without draining it? What pressure PRV should I buy if I need a new one?

Edited March 21, 2008 by blackrose

[steelaway]

I'll see what happens to the PRV after I fit my new waterpump this weekend. The pump's leaking too!

 

Am I right in thinking that as long as I switch off the pump and release the pressure by opening a hot tap, I can remove the PRV from a vertical calorifier without draining it? What pressure PRV should I buy if I need a new one?

 

 

BUGGER!!!

 

 

Alex

[chris w]

I'll see what happens to the PRV after I fit my new waterpump this weekend. The pump's leaking too!

 

Am I right in thinking that as long as I switch off the pump and release the pressure by opening a hot tap, I can remove the PRV from a vertical calorifier without draining it? What pressure PRV should I buy if I need a new one?

 

Correct re draining. PRV should be 3 bar.

 

Chris

[blackrose]

Correct re draining. PRV should be 3 bar.

 

Chris

 

Thanks.

[Eben]

The PRV value is printed on the top of the red knob, they come with the calorifier as reccomended by the manufacturer, values are 1bar, 2bar, 2.5bar and 3 bar in the marine world so have a look at the one you have to get another.

 

The max PRV supplied from most manufacturers is 3bar.

 

Look for lime scale or other items of crud in the valve seating that may stop it from shutting down fully.

 

Has the PRV valve spring weakened? it is rare for these to fail.

 

your water pump pressure should not be too high, otherwise the PRV will blow off sooner than it should as the pressure builds up when the water gets hot.

 

A water pump with about 1.4 bar cut off pressure is better for calorifiers.

 

Julian is correct with the pressure when used as a Accumulator Tank, we set them between 7 and 15 psi when used with a 1.4 bar (20psi) cut off, NO MORE!

 

However, when used as an Expansion Tank we follow the instructions as supplied with the tank, that is: The tank is supplied charged with nitrogen gas at about 2.3 bar (35psi). The pressure should be adjusted to the CUT OFF pressure of the pressurised water pump. This should be done before the tank is installed. N.b, This is just one section of the booklet supplied with the tanks. reccomend you obtain the full copy to fully clear the air!

Edited March 22, 2008 by Eben

[chris w]

A water pump with about 1.4 bar cut off pressure is better for calorifiers.

 

That may or may not be true but commercial pumps for boats have typical cut-off pressures of around 2 bar and cut-in pressures of around 1.5 bar. So a PRV of 3 bar is safe to use with these pumps, in that the pump will never blow the PRV. Of course, in terms of actual PRV value, the ultimate determinant is the maximum safe calorifier pressure as stated on the label.

 

Chris

Edited March 21, 2008 by chris w

[smileypete]

The expansion vessel's pressure (set with a bicycle pump via a valve) should be set a few psi above the PRV release pressure, the latter pressure being around 2.5 - 3 bar typically. So an expansion vessel's pressure would be around 45psi typically.

 

Really? If the expansion vessel pressure is above the PRV release, how will it take up any expansion without the PRV releasing?

 

I'd set the expansion vessel pressure just above pump cut out pressure.

 

cheers,

Pete.

[blackrose]

The PRV value is printed on the top of the red knob, they come with the calorifier as reccomended by the manufacturer, values are 1bar, 2bar, 2.5bar and 3 bar in the marine world so have a look at the one you have to get another.

 

The max PRV supplied from most manufacturers is 3bar.

 

Look for lime scale or other items of crud in the valve seating that may stop it from shutting down fully.

 

Has the PRV valve spring weakened? it is rare for these to fail.

 

your water pump pressure should not be too high, otherwise the PRV will blow off sooner than it should as the pressure builds up when the water gets hot.

 

A water pump with about 1.4 bar cut off pressure is better for calorifiers.

 

My PRV is about 4" below the underside of a kitchen worktop so I can't see what the pressure is until I take it out. Perhaps I'll try a mirror? I didn't really want to remove it until I got another one otherwise I'm without water. I just went out to find one but I forgot it was Good Friday today and all the plumbers merchants are shut. Anyway, it's only a dribble so it's not urgent.

 

My waterpump has a cut in pressure of 20psi and a cut out pressure of 30psi.

[chris w]

Really? If the expansion vessel pressure is above the PRV release, how will it take up any expansion without the PRV releasing?

 

I'd set the expansion vessel pressure just above pump cut out pressure.

 

cheers,

Pete.

 

Whoops, you're absolutely correct - brain not properly engaged (I'll correct it above). The expansion tank pressure should lie somewhere between the pump cut-out pressure (typically 30psi) and the PRV pressure setting (typically around 42psi). So a figure of say 35- 38psi would be in the right range.

 

Chris

[GUMPY]

Whoops, you're absolutely correct - brain not properly engaged (I'll correct it above). The expansion tank pressure should lie somewhere between the pump cut-out pressure (typically 30psi) and the PRV pressure setting (typically around 42psi). So a figure of say 35- 38psi would be in the right range.

 

The expansion tank should be set to about 5psi less than the pump pressure. It is not there solely to absorb the expansion of the hot water, its primary purpose is to smooth the flow of water from the taps which if it is set above the pump pressure it cant do.

You should when turning on a tap be able to get about a litre and a half from the tap before the pump cuts in if the system is set correctly.

 

 

Julian

[blackrose]

The expansion tank should be set to about 5psi less than the pump pressure. It is not there solely to absorb the expansion of the hot water, its primary purpose is to smooth the flow of water from the taps which if it is set above the pump pressure it cant do.

You should when turning on a tap be able to get about a litre and a half from the tap before the pump cuts in if the system is set correctly.

Julian

 

I think you might be confusing the expansion tank with the accumulator tank? The accumulator is set under the pump's cut in pressure for the reason you describe, but the expansion tank in the hot water system (if one is present) is set above the pump's cut out pressure.

[chris w]

The expansion tank should be set to about 5psi less than the pump pressure. It is not there solely to absorb the expansion of the hot water, its primary purpose is to smooth the flow of water from the taps which if it is set above the pump pressure it cant do.

You should when turning on a tap be able to get about a litre and a half from the tap before the pump cuts in if the system is set correctly.

Julian

 

 

WRONG !!!!!!!!!!!!!!!!

 

You're confusing an expansion tank on the calorifier with an accumulator on the cold water system. In essence, the two devices are the same construction but they have different roles to play. The accumulator will be a couple of litres in size while the calorifier expansion tank will be roughly 10% of the calorifier volume and be suitable for hot water use.

 

Chris

 

PS: Sorry Mike, our posts crossed.

Edited March 21, 2008 by chris w

[blackrose]

WRONG !!!!!!!!!!!!!!!!

 

You're confusing an expansion tank on the calorifier with an accumulator on the cold water system. In essence, the two devices are the same construction but they have different roles to play. The accumulator will be a couple of litres in size while the calorifier expansion tank will be roughly 10% of the calorifier volume and be suitable for hot water use.

 

Chris

 

PS: Sorry Mike, our posts crossed.

 

Why is the expansion vessel set at a higher pressure then the pump cut out pressure Chris? I know it's correct but I've forgotten the reason. If you set it too high wouldn't it reduce the capacity for expansion?

[elessina]

I can't see any water coming out of my PRV but there's definately a dribble .....

When first installed we used to get water out of the Pressure relief valve. Our Whale pump was 3bar, pressure relief valve was 3 bar!!!!. The Whale pump has a little grub screw under a QC label that can adjust the pressure (info from Whale direct), this I did, but still water came out of the pressure relief valve.

 

I then removed a non return valve I had installed in the cold water feed to the calorifier (this to stop hot water running back down the cold water supply pipe when the calorifier was hot). No problems with the Pressure relief valve since. This I believe is because the (long length) of cold water supply pipe back to expansion tank from the calorifier acts as a pressure cylinder (when the water gets hot, the pressure increases).

 

Replacement pump is well below 3bar, (I think 1.5bar). I did not want another 3 bar pump!

Our calorifier is also 3 bar design, 5 bar max..... another reason for a lower pressure pump.

Anyone with a Whale 18l/min pump needs to be aware it is 3bar and that most PRV and calorifiers are 3bar rated..... we were not at the time of purchase (3 years ago).

 

Ian

[chris w]

Why is the expansion vessel set at a higher pressure then the pump cut out pressure Chris? I know it's correct but I've forgotten the reason. If you set it too high wouldn't it reduce the capacity for expansion?

 

Because, if it were lower than the pump cut-out pressure eg: say 25psi) , the pump would simply pump up the diaphragm inside the expansion vessel to say 30psi (typical pump cut-out pressure) and there would be no effective room left for true expansion of the hot water.

 

Chris

[blackrose]

Just to clarify the difference between the two tanks: If you click this diagram you can see that although there is an accumulator tank on the cold water supply, there is no expansion vessel on the hot water system.

 

http://www.boatus.com/boattech/casey/16_A.gif

 

The problem with this is that because there is a check valve (NRV) upstream of the calorifier in effect it becomes a pressure vessel. Should the PRV on the calorifier ever get stuck closed something will go bang when the water heats up. The other benefit of the expansion vessel is that it means you don't lose hot water out the PRV unnecessarily.

 

The expansion vessel should either be Td off between the NRV and the calorifier, or off the hot water outlet from the calorifier.

Edited March 21, 2008 by blackrose

[chris w]

Actually, even if there were no NRV installed, there would still be no room for hot water expansion as the cold water accumulator will be full of water already due to the pump's pressure, as in my post just above.

 

Chris

Edited March 21, 2008 by chris w

[blackrose]

Because, if it were lower than the pump cut-out pressure eg: say 25psi) , the pump would simply pump up the diaphragm inside the expansion vessel to say 30psi (typical pump cut-out pressure) and there would be no effective room left for true expansion of the hot water. Chris

Yes, that makes sense, but weren't you suggesting earlier that we set it at 35psi? I'm obviously missing something here...

 

Actually, even if there were no NRV installed, there would still be no room for hot water expansion as the cold water accumulator will be full of water already due to the pump's pressure, as in my post just above.Chris

But if the accumulator was only set at 20psi (below the pump cut in pressure), couldn't it take a bit more pressure? I guess that's not how it works... Edited March 21, 2008 by blackrose

[chris w]

Yes, that makes sense, but weren't you suggesting earlier that we set it at 35psi? I'm obviously missing something here...

 

If we set the expansion vessel at say 35psi, then the pump (30psi) will NOT pump up the diaphragm of the expansion vessel (because the air above the vessel's diaphragm is at a higher pressure, 35psi). So the diaphragm will only move up and accomodate extra water if the pressure increases to above 35psi - as when the water gets hot and expands. Obviously if the water pressure, due to heat, gets over 42psi (3 bar) then the PRV will blow as a safety measure.

 

With regard to the accumulator set at say 15psi or so, its diaphragm is already fully expanded (pushed up) by the fact that the pump cut-out pressure (30psi) is at a higher pressure than the air above the diaphragm (15psi). So the accumulator is already full of water with no room for any more. It's this volume of water which you can run off when you open a tap before the pump cuts in again.

 

Chris

Edited March 21, 2008 by chris w