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

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

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

One of those arrows near pump 2 needs to be a check valve.
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

(OP)
There is a relief valve. I'll look into check valve on Monday. Thanks. Have a good weekend

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

If you have two loops at separate pressures you need two expansion tanks with appropriate controls to keep the base pressure in each loop constant. Your process drawing really doesn't make any sense with showing location of the expansion tanks, as they are essential to you question. I can only assume that you require the 120 psi to keep you fluid from boiling.

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

USAeng,

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

USAeng,

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

(OP)
I will get some answers this week when I get back to the plant. Thanks guys

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

USAeng,

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

(OP)
Thanks Kurt, I will be looking into that tomorrow.

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

Same sort of questions - I realise you simplified the sketch, but closed circuit system usually rely on pressure control / expansion vessels to regulate the pressure and the pumps differential head is often quite small relative to their output pressure. If the thermal fluid can apparently withstand the temperature at 80psi in your second HX, why does it need 120 for the other one?

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

(OP)
Well started looking at things closer. First thing I notice is that I was told max working pressure was 80PSI so I look at the name plate on the heat exchanger - see attached. First the 600kPA converts to about 87PSI so that is a bit higher already. Next, there is no MAWP for the tube side. I assume this is normal for heat exchangers to come with nothing listed here? Thanks

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

(OP)
See attached data sheet from the manufacturer. In the data sheet they have listed both tube and shell side information. They have the max allowable pressure listed at 6 bar = the 87PSI from earlier. The working pressure is left "not known".

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

USAeng,

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

(OP)
Here is a quick sketch with a few more things added. Obviously there is much more in the system -however I only put in things we need for the discussion. Let me know if there is questions or if something is missing. Thanks guys

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

That's starting to make a bit more sense. With that check valve in the outlet of the secondary exhaust HX you should be protected from the primary circuit, but a single check valve is not really sufficient to protect the secondary HX without further protection (relief valve, actuated valve etc).

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

(OP)
There is a relief valve in the thermal fluid line right before the heat exchanger as seen in the picture and the sketch - is this what you refer to?

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

Sure. You don't usually need much of a tank to hold the small relief volumes characteristic of a system that tiny.

you must get smarter than the software you're using.

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

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

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

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

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

(OP)
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

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

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

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

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

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

(OP)
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.

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

(OP)
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

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