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

 

[BigInch]

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.

 

[USAeng]

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

 

[Compositepro]

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.

 

[curtis2004]

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

 

[curtis2004]

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

 

[USAeng]

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

 

[curtis2004]

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

 

[USAeng]

Thanks Kurt, I will be looking into that tomorrow.

 

[LittleInch]

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

 

[USAeng]

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

 

[USAeng]

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

 

[curtis2004]

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

 

[USAeng]

We do have expansion tank before the primary thermal fluid pump. I made new sketch but had to leave work early today so I'll post tomorrow. Here is pic of the heat exchanger for what it's worth

 

[USAeng]

The labels are not exactly right in the picture. The top portion is a bypass for the exhaust.

 

[USAeng]

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

 

[USAeng]

Here is a picture of the heat exchanger with no insulation

 

[LittleInch]

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

 

[USAeng]

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?

 

[BigInch]

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.