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Old 27-05-2013, 05:41   #1
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Replaced faulty T&P relief valve with pressure relief valve on Seaward water heater

After not being used for a six month long winter, the T&P relief valve on a approx. ten years old, 6 gallon Seaward S650 water heater started leaking less-than-lukewarm water after working for about five to ten minutes.
Due to the lack of an expansion tank, I assumed the T&P relief valve was due for replacement.

The valve Seaward provided with this water heater is set at 150 psi (about 10 bar) and 210F. Replacement Seaward valves are available in abundance in the US, but here in Europe there are none to be find. So instead of waiting (and paying, excessively) for the valve from overseas, I figured I'd try and find a similar valve closer to home.

After a little research and visiting a number of stores, I found that the only conventional valves sold here are 'simply' pressure relief valves and are set for 4 bar (about 58 psi). I even got a few chuckles from store employees I showed the Seaward T&P relief valve to, claiming never to have seen one used on a marine water heater.

The confusion, I think, is justifiable. If the point of having a relief valve is to relieve the tank from excess pressure caused by the heating (and expanding) of the water inside, why would you want it to open at both a set pressure buildup and a set water temperature? So why would you nead a T&P relief valve instead of 'just' a pressure relief valve on a small marine water heater?

Anyway, I decided that installing a valve with a significantly lower psi set at the very least wouldn't blow up the boat. So I hooked up the 4 bar pressure relief valve, fired up the water heater and apart from the occasional drip, seems to be doing fine. I have yet to run the engine for a while, so I guess that would be a more revealing test.


If anyone has any thoughts, please share. Also, answer to the prying questions above would be much appreciated!

Thanks in advance.
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Old 27-05-2013, 06:41   #2
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Re: Replaced faulty T&P relief valve with pressure relief valve on Seaward water heat

Most authorities recommend that we NOT depend on a pressure-relief only valve to protect a hot water heating system. Pressure relief alone can't prevent a water heater tank explosion. Pressure relief valves are only intended for protection of tanks that are not heated, because they can't prevent overheating nor reduce temperature.

Search BLEVE EXPLOSIONS for more details.

A T&P valve responds to excessive temperature by expansion of the thermostat which lifts the disc off its seat and discharges overheated water, allowing cooler water to enter the tank and replace the discharged hot water in order to moderate the water temperature. When a valve is relieving water in volume, it is generally due to excessive temperature.

The valve also responds to excessive pressure by discharging water to prevent further pressure increase. If a T&P valve is dripping, rather than relieving in volume, it usually indicates either thermal expansion or debris on the seat, which prevents the valve from closing tightly.

BLEVE EXPLOSIONS (boiling liquid expansion vapour explosions)
It is high temperature that causes an actual "explosion" of a hot water tank, not the pressure itself.
Without high temperature, if a water tank bursts under pressure, water simply squirts out of the tank*.
But if we take those same conditions and add high temperature (over 212F) that occurs in a normally pressurized hot water tank, that is, if we replace cold water with superheated water in the same tank, when the tank ruptures the superheated water, now exposed to the lower pressure of the atmosphere, flashes to steam - a state change that releases enough energy to cause an actual explosion.

* Note: water that is not superheated, that is water under 212F or 100C, has no latent heat energy to cause an explosive expansion, and merely results in a controlled steam pressure energy release.
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Old 27-05-2013, 06:44   #3
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Re: Replaced faulty T&P relief valve with pressure relief valve on Seaward water heat

The purpose of the relief valve is so that the tank doesn't explode . . if the water were to turn to steam the expansion rate is 1760 times the volume that is why both a t&p relief . .example . you shut off the pressure pump but not the heater element and the thermostat failed closed . .obviously failures are rare but that is why the safety . .I would suspect that any water heater t&p relief from the US would meet your needs and not needing to be a specific brand or manufacture
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Old 27-05-2013, 08:08   #4
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Re: Replaced faulty T&P relief valve with pressure relief valve on Seaward water heat

Quote:
Originally Posted by GordMay View Post
BLEVE EXPLOSIONS (boiling liquid expansion vapour explosions)
It is high temperature that causes an actual "explosion" of a hot water tank, not the pressure itself.
Without high temperature, if a water tank bursts under pressure, water simply squirts out of the tank*.
But if we take those same conditions and add high temperature (over 212F) that occurs in a normally pressurized hot water tank, that is, if we replace cold water with superheated water in the same tank, when the tank ruptures the superheated water, now exposed to the lower pressure of the atmosphere, flashes to steam - a state change that releases enough energy to cause an actual explosion.

* Note: water that is not superheated, that is water under 212F or 100C, has no latent heat energy to cause an explosive expansion, and merely results in a controlled steam pressure energy release.
Thank you for your response.

I understand that in this case an 'explosion' occurs when overheated water (which, due to an increased pressure in the tank, can keep on heating up until the temperature exceeds it's 'normal' 212F boiling point) is suddenly exposed to atmospheric pressure by a burst in the tank (caused by, for example, a pressure buildup the tank can't handle) and immediately has to turn into gas.

So now I installed this new pressure-only relief valve which is set at 58 psi. One can assume that the pressure in the tank will not exceed 58 psi, at which water has it's boiling point at approximately 290F. So in turn, you can expect the water not to reach a temperature above 290F. If water in liquid form at this temperature were to somehow reach atmospheric pressure .. kaboom!

The tank, however, is obviously able to handle way more pressure than the PRV allows there to be present (I believe it's tested at 300 psi). The pressure in the tank doesn't even come close to that which would make the tank burst and the temperature doesn't climb above 290F because of said pressure.
In which case, can't the assumption be made that a pressure-only relief valve is sufficient?

Again, thank you for replying. Just keen to get things straight.
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Old 27-05-2013, 08:39   #5
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Re: Replaced faulty T&P relief valve with pressure relief valve on Seaward water heat

A pressure relief by its self its not 100 percent reliable. They tend to get limed up with mineral deposits sometimes and have been know not to work when needed. That's why a T&P is code for all water heaters in north america.

Now with that said, if you did not want to install a thermal expansion tank/accumulator (which is the approved method of dealing with thermal expansion), then what I would do is install a T&P up high at the normal port. But then remove the drain valve, install a nipple and tee with tee facing up and install the 58 psig pressure relief in the up facing hole and the drain valve on the other side of the tee. Best of both worlds.

BTW watts makes a calibrated relief valve, 530C, that is adjustable and can be set at different pressures. Its what I've used in the past where I wanted a specific set point relief (though again not approved as the main T&P relief)

Oh and its not approved to add a tee at the T&P port. This as its important that the temperature element (rod on T&P prelief) extend as far as possible into the top of the tank.

Oh water heater tanks are only tested to 100 or 125 psig. An ASME stamped water heater pressure vessels are sometimes tested to 200 psig, but there is no marine ASME stamped water heaters. Not outside warships anyway. I expect the marine water heaters are not pressure tested to even 100 psig. Glass lined tanks (most landside are) would crack the glass at pressures above 125 psig.
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