Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
(OP)
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...
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
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...
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





RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
A relief valve in the heat exchanger feed might be a good idea.
you must get smarter than the software you're using.
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
It looks to me that you sketch shows only one loop with two pumps.
Can you redraw you sketch with all of your components (boiler, pump, HX, etc.)?
Thanks,
Kurt
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
After reviewing your sketch carefully and reading you OP I have some thoughts to share:
1. What are full system flow through 120 psig pump #1 and flow through 80 psig pump #2?
2. What is estimated pressure drop through the system and what is actual pressure (pressure range) at ti-in point of side system?
Thanks,
Kurt
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
I have done similar project in the past. In my case I have thermal oil system with main oil circulation pumps discharging 60 psig and my waste heat recovery sub-system tied in suction side of main pumps with smaller pumps discharging 30 psig. Although in my case heat exchanger was rated for 150 psig, however it could have been much more lower.
When you say "heat exchanger operating pressure 80 psig and we cannot change this", it is not technically true. Many people in heat exchanger industry confuse design pressure with MAWP. From my previous experience heat exchangers often have additional safety margin built into their "design" or "operating" pressure, because you have only certain wall thickness tubes available.
I suspect that your waste heat recovery heat exchanger construction is similar to air pre-heater, except gases are giving up heat instead of being heated. Liquid/gas heat exchanger is built into a steel box constructed with thin wall tubing in it. Please find out what is tube size, wall thickness, material, and what type of welding/soldering is used to weld tubes.
Regards,
Kurt
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
What is the intended inlet pressure into your first (120 psi) pump?? can it be lowered to reduce the overall pressure?
Simple solution is that suggested by BI, but it is surprising that this design got through a HAZOP or design review without this issue being designed out - have you questioned the designers on this point yet?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
The working pressure must be lower than the max allowable pressure so it appears we must remain below 87PSI in the thermal fluid unless I'm missing something
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
I have doubts that you have shell & tube heat exchanger. It just doesn't make sense to me...
1. Please make a sketch with your heat exchanger showing what is "shell side" and what is "tube side", and where flue gas/thermal oil flows. Can you also find out what is tube size and thickness. Data sheet doesn't have this information.
2. Also give us some more process information. What is the reason for 120 psig pump discharge pressure? What is pump inlet pressure.
3. Do you have expansion tank in your system? Where it is located and what is MAWP?
Thanks,
Kurt
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
The key issue to me is whether the primary circuit actually needs to be at 120 psi or could operate at a lower pressure. If the heating oil in the secondary unit is OK to 260C, does the primary circuit heat it higher?
Could you re-position the secondary unit upstream of your primary circulating pump and put a pressure reducing valve in the main circuit?
There are lots of ways around this, but it needs something to change, either operating pressure or location of the exhaust heater. what are the relative heat input from the exhaust unit and the "fluid heater"?
some idea of flow, temperature and pressures coming into and out of the main bits of equipment you've shown would be good.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
I will have to double check but I think it was already asked of the equipment manufacturer that provided the unit needing the heat who supplied that and the primary pump if we could lower the pressure - the answer I was told from them was no - but I will call personally and ask
We cant reposition the secondary loop since the boiler controls input the heat required based on the inlet temperature - so we want the secondary loop before the boiler
Is there any off-the shelf type tank to route hot oil to that has a level switch built in that we could pipe the relief valves to?
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
you must get smarter than the software you're using.
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
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
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
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
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
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
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
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
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
I'm still not clear why there is no MAWP for the tubes on the nameplate. Did I miss something?
Piping Design Central
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
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
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers
RE: Thermal fluid system pressures - clashing with max operating pressure of heat exchangers