Thermal fluid system pressures - clashing with max operating pressure of heat exchangers 1

[USAeng]

See attached diagram. We have 2 loops in our thermal fluid heating system. One goes to a boiler and the pump produces 120PSI. The other loop gets waste heat from engine exhaust. The exhaust heat exchanger has a listed operating pressure of 80PSI max - we cannot change this.

The design engineers put in some ARI brand pressure regulating valves as shown in the diagram and as seen here

http://www.arivalve.com/pressure_regulating_valves.htm

We have not yet tested the system as we want to understand more first as there are doubts here on the design. It would seem this will work good until a pump turns off - then the heat exchanger will see the 120PSI. Any ideas? Thanks

 

[curtis2004]

USAeng,

Can you tie-in your waste heat recovery system before your system main pumps?
I'm pretty sure that suction side of pumps have operation pressure below 87 psig, and certainly control wise makes a lots of sense.
You didn't respond about MAWP of your expansion tank.

Regards,
Kurt

 

[LittleInch]

USA Eng, maybe you misread my post, but it is essentially the same as Curtis, i.e. if you move the primary pump to after the exhaust HX, but before the main boiler, then you would be operating in the lowest pressure part of the system.

what is the pressure on the main pump inlet?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[USAeng]

Oh ok got it. Unfortunately there are 2 redundant pumps on a very large skid with a huge expansion tank, valves, instruments and safety equipment all piped together. It just isn't feasible to move. Also the inlet/outlet for the exhaust loop it about 1 foot c/c on the supply side to the heater only about 5' from the inlet to the heater and the heater is about 100 yards from the primary pump. I know I didn't give any reference dimension so that wasn't clear sorry.

On another note- I called the manufacturer of the heat exchanger and they said it may be possible to recertify the assembly at the higher pressures. Wouldn't that be the easy way out or what? I'll find out if they can do it tomorrow. Of course it comes with a price, but no new equipment or piping modifications

 

[curtis2004]

USAeng,

According to your pictures, you have by-pass horizontal duct (round) just above heat exchanger. There are two dampers, and heat exchanger (with round shell) installed below. I suspect that flue gases pass through this shell. Inside shell there should be tubing where your thermal fluid is flowing (flue gas duct diameter is bigger in this area than upstream or downstream). If this is the case you should have 87 psig MAWP on "tube side" not "shell side". In this case tubes thickness will determine what pressure can it take. I mentioned that in your heat exchanger data sheet shows material of tubes, but no diameter or wall thickness. If you get diameter and wall thickness you can calculate MAWP based on formulae in Section VIII, Division 1.

There is another alternative when your heat exchanger is constructed like "fire tube" boiler. In that case your thermal fluid is flowing in shell side and flue gases inside tubes. In that case you have to do calculations using Section VIII, Div 1 formulas, for shell and for tubes (because of external pressure on tubes).

That's main reason why I wanted to look inside of your heat exchangers...

Also, I think since your heat exchanger left manufacturer's facility they can't help you re-rate your heat exchanger. It is because ASME code are construction codes, and once vessel left facility for re-rate you have to follow API standards.

Regards,
Kurt

 

[Gator]

I'm still not clear why there is no MAWP for the tubes on the nameplate. Did I miss something?

Piping Design Central

 

[curtis2004]

Gator,

It's because of a mistake. I think German manufacturer made a mistake and it listed tubes side "design pressure", not MAWP.

Once USAeng confirm this and finds out tubes diameter, and wall thickness, and also finds out details of tubes welding we can calculate actual MAWP.

Kurt

 

[USAeng]

the gas is what actually goes through the tubes. They did have more info in the data sheet in the other attachment. Interesting comment on the API for recertification.

 

[USAeng]

The German company wont know until next week whether or not the unit can be recertified. Im holding off any more comments until then but really appreciate all the suggestions and input up to this point. I'll let you guys know what they say and then go from there. Thanks and have a great weekend