PRV maintenance!

[blackrose]

After replacing a leaky pressure release valve on my calorifier I was told to give the cap a twist once every couple of weeks to allow any bits of debris to be blown away before limescale can form around the seal.

 

I never knew this. Any comments?

[Lol]

After replacing a leaky pressure release valve on my calorifier I was told to give the cap a twist once every couple of weeks to allow any bits of debris to be blown away before limescale can form around the seal.

 

I never knew this. Any comments?

 

heard this little gem a lot of times and I'm not 100% sure its the right thing to do, but I suppose if the valve manufacturers recommend it then it must be correct, this usually applies to the valves with the red knurled knob on top

 

if your system has no expansion tank it may lift itself every time water is heated anyway?

 

however why should lime scale form at the seal of the valve, it has more of a tendency to form on surfaces where heat is being exchanged i.e. the coils or immersion elements (i think due to the relatively low temperature of heated coils they are less likely to scale than immersion heater elements), however it can then "grow" but I would have thought that if its grown all the way to the PRV then a leaky seal here is the least of you worries

 

then there is the possibility that you could damage a perfectly healthy seal due to you lifting the trim of the valve then lowering it back onto something washed onto the seal face, i have actually seen this happen with industrial valves

 

my system is fitted with an expansion tank to stop the valve lifting but I am still tempted to "turn the knob" when I am in the engine space but we do inhabit an area of particularly soft water with little chance of scale

 

in summary, Ive waffled on because i have nothing better to do here and the answer is i don't know the answer

[blackrose]

in summary, Ive waffled on because i have nothing better to do here and the answer is i don't know the answer

 

I don't really know either and it was a pump supplier who told me, not the valve manufacturer so he could be wrong too. However, since it's the received wisdom that we should operate our gate valves from time to time to stop them seizing, then why not PRVs too?

[Lol]

I don't really know either and it was a pump supplier who told me, not the valve manufacturer so he could be wrong too. However, since it's the received wisdom that we should operate our gate valves from time to time to stop them seizing, then why not PRVs too?

 

more importantly with any kind of rotary valve is to not completely open it, open it till it stops then close it half a turn to take the tension out of the threads

 

also with gate valves, yes do operate them every now and then but i wouldnt close them tight, wind just short of closed then wind back to open, this is to minimise damage to the machined faces of the valve from debris so when you do want it to seal it has more chance

[chris w]

heard this little gem a lot of times and I'm not 100% sure its the right thing to do, but I suppose if the valve manufacturers recommend it then it must be correct, this usually applies to the valves with the red knurled knob on top

 

if your system has no expansion tank it may lift itself every time water is heated anyway?

 

however why should lime scale form at the seal of the valve, it has more of a tendency to form on surfaces where heat is being exchanged

 

 

Calcium etc will precipitate out of water at around 70-75C. The water in your calorifier will be at around 85C if heated by the engine so precipitation will occur. (That's why you shouldn't run your home heating tank above 70C.) Further, the continual dribbling of hot water through the PRV on calorifiers that don't have an expansion tank will also encourage calcium deposits to form there which may interfere with the proper sealing of the PRV.

 

It is for this reason that it is a "good thing" to occasionally turn the PRV's knob a few times periodically to break up any calcium deposits. It's also a good thing anyway to do it to check that the PRV is not seized. The latter condition would result in a calorifier failure if pressure (due to water expansion) were not able to escape when the water is heated.

 

It's also a good opportunity to check all the pipes in that area at the same time to make sure none are weeping, or even leaking, before real damage is caused.

 

Chris

[PeterF]

however why should lime scale form at the seal of the valve, it has more of a tendency to form on surfaces where heat is being exchanged i.e. the coils or immersion elements (i think due to the relatively low temperature of heated coils they are less likely to scale than immersion heater elements), however it can then "grow" but I would have thought that if its grown all the way to the PRV then a leaky seal here is the least of you worries

Lol,

 

If the PRV is weeping a little then the water will evaporate as it weeps because it is warm and this will leave deposits of lime scale behind as the water evaporates.

 

PeterF.

[Tony Brooks]

more importantly with any kind of rotary valve is to not completely open it, open it till it stops then close it half a turn to take the tension out of the threads

 

also with gate valves, yes do operate them every now and then but i wouldnt close them tight, wind just short of closed then wind back to open, this is to minimise damage to the machined faces of the valve from debris so when you do want it to seal it has more chance

 

 

Its not a rotary valve.

 

Its an ordinary spring loaded "tap washer" type valve with a mechanism that lifts the jumper and drops it again as you twist the cap.

 

Colour of the cap is not important. Mine are black.

[Keeping Up]

But if there IS an expansion tank and the valve doesn't dribble, surely there's no reason to assume that the sealing faces will have any scale on them?

[Lol]

Its not a rotary valve.

 

Its an ordinary spring loaded "tap washer" type valve with a mechanism that lifts the jumper and drops it again as you twist the cap.

 

Colour of the cap is not important. Mine are black.

 

all of the following is in IMHO.........

 

Tony

 

my answer was written to a wider audience than engineers alone, a gate valve (if you read the post that is what i was talking about) is rotary to the layman in that you rotate a handle to actuate even though this is then transfered via a thread to a perpendicular to flow linear motion of the tapered gate or disc. Most valve seizing occurs at the thread not the internals except where as with a gate valve when it is closed the faces of the disc and the valve body can seize together

 

Quite right they can be different colours on the top! but i think you will find red is by far the most common, maybe we should poll the forum on knob colour

 

Chris W

 

as for precipitation of Calcium salts (not Calcium) unusually these are inversely soluble (more heat less soluble) and can in certain situations precipitate at temperatures below 80degC, in the bulk water and is more likely to need a "key" to start it such as a heated surface, this as I said is less likely to be the coils and more likely the imersion heater elements which can be easily 100degC+

 

Peter F

 

good point and not one I had considered, if it weeps and precipitation forms on the seat/trim then it could continually weep and may benefit from a blast to break it up

 

my mind on this works a little differently in that in the real world of pressure relief or safety valves the last thing you want is them weeping, indeed where I'm working right know if a valve has actuated and passed water it has to be re-calibrated, if these boat valves have been fitted to weep due to expansion its more an unloader than a PRV (or PSV)

 

this is why I fitted two accumulators to my system which is plumbed with no check valve between the hot and cold so that pump pulsations are taken by the lower set accumulator and then heat expansion is taken by the upper set one through the whole system

 

In conclusion (but I'm still not 100% in my own mind about this) if you have an accumulator on the hot this is good for the valve and will mean it should never lift and as the mechanism is dry and the seal is elastomeric to metal should never seize, if you don't have an accumulator and the valve weeps then blast it, or fit an accumulator

 

oh by the way you will probably find if you have a fairly modern boiler at home it has the same kind of valve fitted and my Baxi makes no mention of blasting it!!!!

[Tusses]

Lol ...

 

did you actually type anything ? all I can see is some gentle bouncing in the top left ?

 

[chris w]

as for precipitation of Calcium salts (not Calcium) unusually these are inversely soluble (more heat less soluble) and can in certain situations precipitate at temperatures below 80degC, in the bulk water and is more likely to need a "key" to start it such as a heated surface, this as I said is less likely to be the coils and more likely the imersion heater elements which can be easily 100degC+

 

Yes I know it's calcium SALTS. I ws trying to keep it simple. Recommendation for home boilers is never to have the hot water cylinder set above 70C to help prevent precipitation.

 

oh by the way you will probably find if you have a fairly modern boiler at home it has the same kind of valve fitted and my Baxi makes no mention of blasting it!!!!

 

I have a totally pressurised system at home. Both sides are pressurised too, viz: the boilers are pressurised and have a very large expansion vessel fitted and the domestic hot water is pressurised and has a sealed Megaflow vessel. The instructions for both of these pressure vessels advocates rotating the PRV knobs on them a few times once a month.

 

Chris

[Tony Brooks]

all of the following is in IMHO.........

 

Tony

 

my answer was written to a wider audience than engineers alone, a gate valve (if you read the post that is what i was talking about) is rotary to the layman in that you rotate a handle to actuate even though this is then transfered via a thread to a perpendicular to flow linear motion of the tapered gate or disc. Most valve seizing occurs at the thread not the internals except where as with a gate valve when it is closed the faces of the disc and the valve body can seize together

 

Quite right they can be different colours on the top! but i think you will find red is by far the most common, maybe we should poll the forum on knob colour

 

Snip

 

<quote>

 

Sorry, I thought the subject was the Pressure Relief Valve. I had not noticed the change to gate valves. I agree modern gate valves usually have red knobs, but the same can not be said about the PRVs used on boat plumbing systems, although some do.

[Lol]

Tony, sorry to be pedantic but i was talking about the PRVs having red tops, most ive seen are red

 

but yes you are correct most gate valves in general use have red handles i believe this is becase they are accepted for potable water use whereas yellow handled valves are usually gas certified

 

 

Chris, no need to shout I have just re-read what i said and it it a little condescending - sorry

 

however if i wanted to be clever i would have pointed out that at heated surfaces it can be Calcium or even Magnesium salts and that the point about water evaporating and leaving deposits this is not strictly speaking "lime scale" but is a combination of all the dissolved salts that were in the water and .....oh shut up Lol

 

Tusses - good yeah

[blackrose]

this is why I fitted two accumulators to my system which is plumbed with no check valve between the hot and cold so that pump pulsations are taken by the lower set accumulator and then heat expansion is taken by the upper set one through the whole system

 

In conclusion (but I'm still not 100% in my own mind about this) if you have an accumulator on the hot this is good for the valve and will mean it should never lift and as the mechanism is dry and the seal is elastomeric to metal should never seize, if you don't have an accumulator and the valve weeps then blast it, or fit an accumulator

 

Lol, although I think we all knew what you meant, since you were being pedantic I hope you don't mind if I am too.

 

An "accumulator" type tank fitted to the hot water system (most commonly between the NRV and calorifier, or downstream of the calorifier), is not actually operating in the same way as a cold water accumulator tank and is more correctly called a hot water expansion vessel.

[Lol]

Lol, although I think we all knew what you meant, since you were being pedantic I hope you don't mind if I am too.

 

An "accumulator" type tank fitted to the hot water system (most commonly between the NRV and calorifier, or downstream of the calorifier), is not actually operating in the same way as a cold water accumulator tank and is more correctly called a hot water expansion vessel.

 

correctly should read commonly

 

more pedantry, they are one and the same and do eactly the same job for different reasons, they accumulate pressure, one from the pump the other from heat induced expansion and are correctly called accumulators (or hydropneumatic accumulators) they are designed specifically for the storage (accumulation) of liquid under pressure retaining the imparted energy for re-use

[blackrose]

correctly should read commonly

 

more pedantry, they are one and the same and do eactly the same job for different reasons, they accumulate pressure, one from the pump the other from heat induced expansion and are correctly called accumulators (or hydropneumatic accumulators) they are designed specifically for the storage (accumulation) of liquid under pressure retaining the imparted energy for re-use

 

No, by correctly I meant correctly! A vessel used to accommodate the themal expansion (not accumulation) of liquids is correctly known an expansion vessel, not an accumulator tank, and that's even the case if an accumulator tank has been bought but used as an expansion vessel. They have completely different functions. An accumulator tank stores imparted energy for re-use as you say, but the primary function an expansion vessel is to accommodate thermal expansion.

Edited April 4, 2008 by blackrose

[tomsk]

Yes I know it's calcium SALTS. I ws trying to keep it simple. Recommendation for home boilers is never to have the hot water cylinder set above 70C to help prevent precipitation.

 

 

 

I have a totally pressurised system at home. Both sides are pressurised too, viz: the boilers are pressurised and have a very large expansion vessel fitted and the domestic hot water is pressurised and has a sealed Megaflow vessel. The instructions for both of these pressure vessels advocates rotating the PRV knobs on them a few times once a month.

 

Chris

 

Had any probs with your Megaflo yet?

[tomsk]

No, by correctly I meant correctly! A vessel used to accommodate the themal expansion (not accumulation) of liquids is correctly known an expansion vessel, not an accumulator tank, and that's even the case if an accumulator tank has been bought but used as an expansion vessel. They have completely different functions. An accumulator tank stores imparted energy for re-use as you say, but the primary function an expansion vessel is to accommodate thermal expansion.

 

The Weekly Pedants Award goes to Rosie for the above.

[blackrose]

The Weekly Pedants Award goes to Rosie for the above.

 

And the annual tossers award goes to... this bloke

(amongst others) Edited April 4, 2008 by blackrose

[Lol]

No, by correctly I meant correctly! A vessel used to accommodate the themal expansion (not accumulation) of liquids is correctly known an expansion vessel, not an accumulator tank, and that's even the case if an accumulator tank has been bought but used as an expansion vessel. They have completely different functions. An accumulator tank stores imparted energy for re-use as you say, but the primary function an expansion vessel is to accommodate thermal expansion.

 

go to the ironmongers and buy two nails, knock one in the wall and hang a picture on it, its a picture hanger, knock the other one in the wall and hang a coat on it, its a coat hanger - both still just nails..........

[blackrose]

go to the ironmongers and buy two nails, knock one in the wall and hang a picture on it, its a picture hanger, knock the other one in the wall and hang a coat on it, its a coat hanger - both still just nails..........

 

A useful lesson for you - if you're going to be a pedant, get it right!

 

You can certainly buy an accumulator tank for use as a hot water exapansion vessel, but since you were describing a freshwater system on a boat you're liable to confuse people if you refer to both tanks as accumulators when in use.

Edited April 4, 2008 by blackrose

[Lol]

A useful lesson for you - if you're going to be a pedant, get it right!

 

You can certainly buy an accumulator tank for use as a hot water exapansion vessel, but since you were describing a freshwater system on a boat you're liable to confuse people if you refer to both tanks as accumulators when in use.

 

fair enough

[magnetman]

I think the correct term is Pressure -Relief- Valve, not -Release- as it is designed to 'relieve' a situation which could otherwise cause aggrevation. Mine's got a red knob. I may be wrong.

[blackrose]

I think the correct term is Pressure -Relief- Valve, not -Release- as it is designed to 'relieve' a situation which could otherwise cause aggrevation. Mine's got a red knob. I may be wrong.

 

I think you're probably right.

[magnetman]

Cool