Unbalanced heat load (CCPS GPREH2 trim)

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Unbalanced heat load (CCPS GPREH2 trim)

Thursday, June 25, 2026

The proper evaluation of some causes of overpressure for nonreactive systems involves consideration of both excess material entering/leaving the system and heat transfer within the system. Many causes of overpressure affecting distillation towers require this consideration. For complex operations, process simulators are commonly employed to assist with the heat and material balances as they have the added advantage of being capable of determining the thermophysical properties of the fluids involved.

Unbalanced heat load method. One common estimation technique employed is the unbalanced heat load method, where the heat and material balance on the system is used to determine how much mass and energy needs to be removed from the system via the venting relief stream in order to satisfy the balances. The overall system material and energy balances presented in CCPS GPREH 2nd Edition §3.21 can be simplified as follows, with any accumulation of fluid within the system accounted for as separate terms:

∑WF=∑WE+∑WACC+WT+W\sum W_{F} = \sum W_{E} + \sum W_{\text{ACC}} + W_{T} + W

 

QF+∑WFHF=QE+∑WEHE+∑WACCHACC+WT(Hg−Hgf)+WHgQ_{F} + \sum {W_{F}H_{F}} = Q_{E} + \sum {W_{E}H_{E}} + \sum {W_{\text{ACC}}H_{\text{ACC}}} + W_{T}\left( H_{g} – H_{\text{gf}} \right) + WH_{g}

There are two accumulation terms in the equations above. The first is WT, which hypothetically represents the accumulation of liquid absorbing the unbalanced energy, part of which is vaporizing to produce the relieving fluid. The second is WACC, which represents the accumulation of liquid elsewhere in the tower. For a simple distillation system having a single feed, distillate product stream, and bottoms product stream, the accumulation of liquid elsewhere in the tower (specifically at the bottom) may occur if the bottoms product stream flow was zero or reduced due to the particulars of the overpressure scenario. The choice of fluid and accumulation rate can have a significant impact on the calculated relieving rate, especially for accumulations associated with other feeds or draws, such as side draws in complex towers.2,3 In practice for distillation towers, the WACC term usually only represents the bottoms fluid that may be accumulating at the bottom of the tower.

Combining the equations on the hypothetical top accumulation term, WT, and solving for the venting requirement, W, noting that the determination of the total energies associated with a class of streams is the product of the flow rates and the specific enthalpies of those streams:

Wmin⁡=1Hgf(ΔQ+∑WxHx−(∑WF−∑WE−∑WACC)(Hg−Hgf))W_{\min} = \frac{1}{H_{\text{gf}}}\left( \Delta Q + \sum {W_{x}H_{x}} – \left( \sum W_{F} – \sum W_{E} – \sum W_{\text{ACC}} \right)\left( H_{g} – H_{\text{gf}} \right) \right)

 

ΔQ=QF−QE\Delta Q = Q_{F} – Q_{E}
∑WxHx=∑WFHF−∑WEHE−∑WACCHACC\sum {W_{x}H_{x}} = \sum {W_{F}H_{F}} – \sum {W_{E}H_{E}} – \sum {W_{\text{ACC}}H_{\text{ACC}}}

where the subscript “F” represents feeds to the system, the subscript “E” represents products from the system, WACC is the rate of material accumulation within the system, WT is the accumulation of liquid remaining after vaporization of the venting requirement, QF is the sum of the heat added to the system, and QE is the sum of the heat removed from the system.

The unbalanced energy in the system is applied to a representative fluid, and the portion of that fluid which is vaporized is assumed to completely disengage and be relieved. In practice for distillation towers, this representative fluid is usually taken to be the distillate product. Note that the system is expected to be far from the thermodynamic critical point, and the specific volume correction factor is ignored as the factor is expected to be close to unity. Also, performing the heat and material balances for the system during normal operation is recommended before attempting the heat and material balances for the system during the cause of overpressure.

One challenge to the analysis above is the specification of the flow rates and enthalpies of the streams entering or leaving the system based on the selected balance envelope, as the specification requires some knowledge of how these streams are going to behave in response to the overpressure event. Engineering judgment is required to determine the proper specifications for these streams, and consideration should be given to the response of the system at the elevated pressures. An additional challenge is encountered for systems where any of the feed streams are noncondensable gases or superheated vapors, as the simplifications imposed include the fluids being in thermodynamic equilibrium. A typical approach to handle these streams is to ensure the total relieving requirement includes the noncondensable gases being added.

As indicated above, the analysis depends on the selection of the envelope on which to perform the heat and material balance. The interrelationship between the envelope selected and the assumptions made regarding the specification of flow rates and enthalpies of the streams crossing those envelope boundaries should be understood as the calculation results can be very sensitive to this interrelationship. A sensitivity analysis of the assumptions used may be useful, including the assumptions made regarding the accumulation terms as indicated above.

Process simulation. To perform a similar evaluation in a process simulator for a distillation tower, the distillation tower unit operation is converted into an absorber unit operation in order to handle the potential for trays drying out during the analysis. Since many of the causes of overpressure involve loss of cooling, the overhead condenser is extracted from the simulation as a separate unit operation (typical distillation tower unit operations contain both the overhead condenser and reboiler as integral to the unit operation), and the streams to and from the tower are specified. The material accumulation term is essentially the flow out of the bottom of the absorber. The vapor flow out of the top of the absorber is the venting requirement, unless some credit is taken for the operation of the overhead cooling or continued flow to the downstream system (e.g. credit for partial condensing systems where some vapor outflow is expected to continue).

Blog series information. This blog is part of a series on the proposed updates to the CCPS Guidelines 2nd edition §3.3 Venting Requirements for Nonreacting Cases that were removed during final editing. See the general CCPS Guidelines for Pressure Relief and Effluent Handling 2nd Edition review for more information.

 

[1] AIChE Center for Chemical Process Safety. “CCPS Guidelines for Pressure Relief and Effluent Handling Systems”. 2nd Edition, 2017; New Jersey: John Wiley & Sons, Inc.

[2] Sengupta M, Staats FY. Calculations improved for relief-valve load. Oil and Gas Journal. May 22, 1978. 74-76.

[3] Sengupta M, Staats FY. A new approach to relief valve load calculations. Hydrocarbon Processing. May 1978. 160-162.

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