Effect of DP on PVs
Effect of DP on PVs
(OP)
Most heat exchangers or pressure vessels that I have come into contact with are design for either full shell and/or full tube side pressures.
In the case of heat exchangers when fouling is concerned, can a high differential pressure damage tube-to-tubesheet joints or any other location?
(By differential pressure I mean with respect to one side only, so on the tubeside, the inlet is say 100 psig and the outlet is 80 psig.)
In the case of heat exchangers when fouling is concerned, can a high differential pressure damage tube-to-tubesheet joints or any other location?
(By differential pressure I mean with respect to one side only, so on the tubeside, the inlet is say 100 psig and the outlet is 80 psig.)





RE: Effect of DP on PVs
But how many times has a partition plate been torn off with nobody ever thinking "I wonder what the stress in the channel was when this tore off?" How many thermal efficiency losses are completely attributed to "fouling" when in fact some of the efficiency loss is simply tubeside fluid bypassing the tubes due to a partition plate failure?
RE: Effect of DP on PVs
Regards,
Mike
The problem with sloppy work is that the supply FAR EXCEEDS the demand
RE: Effect of DP on PVs
Do they normally set the inlet and outlet temperatures to be different with respect to mechanical design to determine thermal stress ? Most I've seen just set one temperature and pressure for thickness calculation.
Now the actual designer may actually do all that and analyze with FEA in their internal documents but that's probably propriety information that they don't give with their units.
RE: Effect of DP on PVs
Regards,
Mike
The problem with sloppy work is that the supply FAR EXCEEDS the demand
RE: Effect of DP on PVs
But like I said, most I see don't show the thermal stresses part as the OEM doesn't normally send that part.
RE: Effect of DP on PVs
What kind of exchangers are you talking about? Floating head, U-tube, something else? What is your Code of construction? Do they fall under TEMA?
Regards,
Mike
The problem with sloppy work is that the supply FAR EXCEEDS the demand
RE: Effect of DP on PVs
RE: Effect of DP on PVs
Well, I just re-read his last sentence: "(By differential pressure I mean with respect to one side only, so on the tubeside, the inlet is say 100 psig and the outlet is 80 psig.)"
I guess I took it to mean, well, I'm now not sure what.
Could be about pressure drop across one side only, in which case the pass plate is a concern on the tubeside, a long baffle would also be a concern on the shell side. Can't think of anything else offhand that I'd be too worried about.
Really, I don't often see a specified allowable pressure drop over about 10 psig. Operate over that allowable and the unit performance is affected due to (I suppose) pump limitations, flow rates being off and so forth. This is the process engineers' side of things, not really mine :)
And yeah, the temperature gradient from one side of the channel to the other is not routinely considered during design of the run-of-the-mill shell & tube exchanger.
Well, perhaps we will learn more if we stay tuned :)
Regards,
Mike
The problem with sloppy work is that the supply FAR EXCEEDS the demand
RE: Effect of DP on PVs
Let me try to explain better.
Say we have a brand new heat exchanger (fixed tubesheets) and I'm only considering the tubeside for now.
Let's say the tubes and tubesheet is operating to 80 psig and 100F. So my documentation shows me mechanical calculations for the tubes and tubesheet based on these to both ASME 8 Div.1 and TEMA.
Now my exchanger has been in service for a year and heavy fouling is occurring so now my inlet is getting clogged such that the inlet pressre is now 120 psig and my outlet is now 75 psig. So across the tubeside there is a DP = 120-75 = 45 psi.
My question is, while 45 psi may not cause a tube rupture or the tubesheet to crack based on pressure alone but can it have any other detrimental effects such as buckling of tubes due to impingement on baffles, damage to the tube-to-tubesheet joints, etc.
My second question, is that if my HX is designed to TEMA, I see it calculates thermal differential expansion for the shell and tubes and tells me which is higher. Can I use this as basis to determine that if I calculate a tube expansion greater than this that I will damage my tubes ?
And well I guess now based on my second question, if I have one side as a floating tubesheet if I calculate a thermal expansion to be greater than the stop on the floating head side, can I use this as a basis to say I need to clean my exchanger ?
(Let's pretend that I have some way to measure all these temperatures accurately)
RE: Effect of DP on PVs
Of more (much more) interest, especially in the case of a fixed tubesheet exchanger, is what is happening with the fluid temperatures. Fouling is a thermal resistance. It prevents heat transfer from the hot side to the cold side. Hot side exit temperature will approach the inlet temperature. The same is true for the cold side. In the extreme, the inlet and outlet temperatures for each side are equal, no heat is transferred.
For floating head or U-tube exchangers, no big deal, since differential thermal expansion between the tubeside and shell side is not a design consideration, assuming clearances are such that no "collisions" occur within the exchanger. By the way, I've never seen "stops" on a floating head exchanger, got an example to share?
For fixed tubesheet exchangers, differential thermal expansion is a big deal, due to the stresses imposed on the shell, tubesheet, tubes and tubesheet joints, both tensile and compressive. The differential expansion is almost entirely due to the difference in the so called "mean metal" temperatures of each side. These are a representation of an "average" of the inlet and outlet temperatures. The differential is also a pretty strong function of the metallurgy of the exchanger. Pressures alone have a negligible effect.
So, for a given fouling, and supposing you know the associated flowrates and so forth, a new operating point can be calculated and the mean metal temperatures obtained. The fixed tubesheet design can be evaluated to see if the imposed stresses are within allowables. If so, operate away.
As you said, TEMA gives a calculation for differential thermal expansion as does Part UHX. These are based on metallurgy and the mean metal temperature for each side of the exchanger. As the operating point changes, so do the MMT's and the differential expansion. So in general, no, you cannot use the differential expansion as a basis for when to clean.
The key measure is the performance of the exchanger. When it no longer transfers the required amount of heat whatever is the cause, fouling or not, must be addressed. And by the way, fluid inlet and outlet temperatures are routinely monitored with acceptable accuracy on the vast majority of operating heat exchangers. That together with known flowrates is the ONLY basis for calculating performance.
Enough for tonite :)
Regards,
Mike
The problem with sloppy work is that the supply FAR EXCEEDS the demand
RE: Effect of DP on PVs
I may have mixed a common term used here with the actual term with when I referred to "floating". I was mainly referring to one end being allowed to slide a set distance (so one end is fixed onto a concrete base and the other end is allowed to expand a fixed distance calculated by the designer). Sorry about that mix up.
Which then like you said, it shouldn't be a problem as it is still allowed to move.
But yes we do use heat exchanger performance but due to fouling, some people tend to get panicked with overheating a bundle because of the fouling. (which again, as you said, once the performance is unaffected it should be fine).
RE: Effect of DP on PVs
(a) Due to excessive pressure drop / fouling on the tubeside, to keep up HX performance, you have to increase shellside flow to the extent that you have a concern on what mean metal temps may be on the tubes and the tubesheet ?
AND
(b)That you have a HX with different design temp for shellside and tubesides?
RE: Effect of DP on PVs
Due to pressure drop as a result of fouling, there would be a DP of around 20-30 psi while the design data might say the allowable is 5 psi. So due to higher DP, less flow is allowed through the unit so what tends to happen is that to keep the unit online they would decrease the flow (this HX is in a loop so the flow through the tubeside = flow through shellside)
(the tube metal temperature is another issue that may be extremely complex to determine)
My initial question was mainly about DP damaging mechanical components. But it can be extended to temperature differential across the tubes also causing damage due to higher mean metal temps due to fouling.
RE: Effect of DP on PVs
Shouldnt take much effort to find out the current tubeside fouling coeff and the new tube metal wall temp at the hot end of this HX, since you have the flows and temps on both sides of the HX in the current fouled condition.