Design Press/temp - Low Press & High Temp, OR High Press & Low Temp??

[GaTechTheron]

Degree: Mechanical Engineer
Location: Houston, TX USA
Experience: Rotating Equipment Design & modular skid Fabricator (4yrs), EPC (2yrs), Owner (4 Years)
Position: PM

Question: So I have seen this scenario a few times in my career, but would like to better understand what a pressure vessel should be designed upon in order to check engineers. I'm currently working to debottleneck a distillation column on an existing unit. We are evaluating the hot oil system, and the process engineers indicate that the design temp/pressure is not appropriate for some equipment in the system.

During start up of a hot oil system, the SG of the heating medium we use is 1.0sg, but ambient temperatures. When at full operation, the SG of the fluid is then 0.8sg, but temperatures are elevated. For argument sake, I'm going to exaggerate the pressure during start up, because I'm trying to understand the principle here... otherwise actual pressure in this case is 300[psig]

Start Up:
400[psig] max pump discharge pressure
90[F] heating medium temperature

Operating:
200[psig] max pump discharge pressure
600[F] heating medium temperature

For design of heat exchanges downstream of the pump, how do you determine what the design temperature/pressure is? Do you take the higher value from both cases, and combine for your design temp/press? Do you determine which individual case is "worse case," and design for this only?

 

[XL83NL]

In general depends on the design code, which wil lexactly spell this out out, but in short; whichever leads to the highest required thickness.

ASME B31.3 (for piping) for example is very explicit in this matter;

301.2 Design Pressure
301.2.1 General
(a) The design pressure of each component in a piping
system shall be not less than the pressure at the most
severe condition of coincident internal or external pressure
and temperature (minimum or maximum) expected
during service, except as provided in para. 302.2.4.
(b) The most severe condition is that which results
in the greatest required component thickness and the
highest component rating.
(c) When more than one set of pressure–temperature
conditions exist for a piping system, the conditions governing
the rating of components conforming to listed
standards may differ from the conditions governing the
rating of components designed in accordance with
para. 304.
(d) When a pipe is separated into individualized pressure-
containing chambers (including jacketed piping,
blanks, etc.), the partition wall shall be designed on the
basis of the most severe coincident temperature (minimum
or maximum) and differential pressure between
the adjoining chambers expected during service, except
as provided in para. 302.2.4.

...
..
..

301.3 Design Temperature
The design temperature of each component in a piping
system is the temperature at which, under the coincident
pressure, the greatest thickness or highest component
rating is required in accordance with para. 301.2. (To
satisfy the requirements of para. 301.2, different components
in the same piping system may have different
design temperatures.)
In establishing design temperatures, consider at least
the fluid temperatures, ambient temperatures, solar
radiation, heating or cooling medium temperatures, and
the applicable provisions of paras. 301.3.2, 301.3.3, and
301.3.4.

ASME VIII Div 1, which is for vessels, states;

UG-20 DESIGN TEMPERATURE
(a) Maximum. Except as required in UW-2(d)(3), the
maximum temperature used in design shall be not less
than the mean metal temperature (through the thickness)
expected under operating conditions for the part considered
(see 3-2). If necessary, the metal temperature shall
be determined by computation or by measurement from
equipment in service under equivalent operating conditions.
See also U-2(a).
(b)...
(c)...
(d)...
...

UG-21 DESIGN PRESSURE
Each element of a pressure vessel shall be designed for
at least the most severe condition of coincident pressure
(including coincident static head in the operating position)
and temperature expected in normal operation.
For this condition, the maximum difference in pressure
between the inside and outside of a vessel, or between
any two chambers of a combination unit, shall be considered
[see UG-98 and 3-2]. See also U-2(a).

Please note UG-20 has more statements, but most of these dont pertain to your query. Hope this helps.

 

[georgeverghese]

The HX and HO system design should take each case individually into account, and not take the severest of both cases to be acting coincidentally (i.e 300psig at 600degF in this case). This cannot happen in reality at a HO heat exchanger in any case.

Obviously the max pressure in each of these 2 cases should be based on the pump running at full speed at dead head conditions (due to failure of the FSHH to trigger a pump trip), and with pump suction pressure at its max possible (which would be the PSHH setting at the upstream HO drum). While the temp in the normal operating case (which you say is 600degF) should be the max operating temp (i.e setting for the TSHH on the hot oil heater exit).

Pls note the design pressure selection of the hot oil side of the reboiler/ HX should also take into account whether the process designer deems it necessary to accomodate a tube rupture overpressure scenario ( if this is applicable) at the HX itself, or allow for this to to be accomodated at the HO expansion drum vapor space PSV.

 

[Trestala]

The key here is worst coincident pressure-temperature (plus static head).
You have to evaluate them individually and see which would govern.

Would there be a chance that the maximum pressure would coincide with maximum temperature? If no, then you do not need to combine the highest pressure with the highest temperature.