Reply To: T&P relief valve discharge line pressures

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SylvanLMP
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In reply to message posted by Jerry Peck:
If a T&P reilef valves opens at 150 psi, the pressure at the valve opening is 150 psi, but what is the pressure in the discharge line while the valve is open, and, what will the pressure be at the end of a 10′ long discharge line with 4 elbows (two near the T&P valve and two near the end of the end of the discharge line)?

I.e., is there a pressure loss per foot of pipe (assuming 3/4″ pipe) and per elbow and approximately what would it be?

Thank you in advance.

A curious non-plumber.


Hi Jerry,

Your question is very valid and unfortunately contrary to ASME section IV code and NBBI code and most REAL plumbing codes

Your 4 ell bows on the discharge piping is ILLEGAL

A 10 ft run is very questionable at best.

As a Certified Unfired pressure vessel and low pressure boiler inspector (up to 15 PSI steam or 160 PSI water @ 250 degrees)

They frown upon excessive runs of piping and having any fitting installed between a relief Or safety Or T&P is contrary to most good piping applications.

The real concern is the back pressures that can result (friction losses) and the possibility of piping having its internal area restricted.

You had not even think of having any relieving line pitch upwards from the boiler/pressure vessel as this would create a pocket where sediment or water could build up and thus severely reduce the relieving capacity of the device (valve)

There are some really great books that every single LMP or inspector should read regarding relief piping sizing.

The NFPA- AGA (now CS) ASME section IV and several other sections dealing with proper piping arrangements.

The National Board is a fantastic place to get easy to read directions on piping lay outs.

Now for a point of information.

Some of the United States plumbing codes I have read are geared for a 6th grade reading level at best.

This is not a put down it is fact.

If you see what is being allowed to be installed in some states and the non skilled being allowed to be inspectors or installers the local codes have become so diluted they are just about useless.

The home builders are now the code writers and the unemployables are now doing tract house plumbing and under slab heating.

Therefore I would strongly suggest you learn from the Professional Codes still available like the American Society of Mechanical Engineers Or the National Board of Boiler Inspectors.

Today’s “American plumbing” publications are nothing more then a place for the weak in the mind to learn to do pricing systems rather then actually learn a decent code.

The only thing that will save the people from inept installers and non knowing “inspectors” is the tort laws.

Thankfully I was able to serve as an expert witness and was able to hang out to dry many bad plumbing/heating and inspection services who failed to take proper measures and follow minimum code standards.

As a certified inspector seeing 4 elbows installed on a 3/4″ line I would look to go against someone’s license or at least fine them so they would take more interest in doing the correct job the next time.

Criminal neglect is a great clause on the law books and it should be used much more often then it is.

Giving someone a fine is one thing BUT going after them personally is a great way to discourage the incompetent ones from dabbling in fields they have no clue about.

There are great codes for a reason now all we have to do is have someone read them to the unknowing who dabble in the trades.

Jerry, if you like E mail me as I have a list where you can meet many highly qualified LICENSED Engineers and Licensed Master Plumbers,Gasfitters etc. from almost every state and several countries.

I have one of the best plumbing/ fire supression inspectors in the entire country on my list.

He is the inspector inspectors go for information

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