How to Determine the Fire Case PSV Relieving Temperature
Lokesh Kumar Singh
BEng CEng MIChemE, Lead Process Engineer
China Petroleum Abu Dhabi

This is one of the most confusing and contradicting concept of how to determine the accurate relieving temperature and how to develop HYSYS simulation model to predict it accurate. There are many ways for process engineers to use this concept in the hydrocarbon design industry. But very few methods could predict the accurate relieving temperature. To develop the HYSYS simulation model for determining the most accurate relieving temperature, we need to build HYSYS model in such a way that it represents the exact same situation which happens during fire, specially across an equipment on which PSV is mounted. During fire case, equipment is isolated by automatically closing inlet and outlet valves across a vessel. Now there is a trapped inventory in the vessel closed loop, which consists of some piping volume and the vessel volume. There may be two scenarios for calculating the inventory volume trapped in the equipment loop - normal liquid level operating case and maximum liquid level operating case. The inlet flow rate is not dependent on building a HYSYS simulation model whether plant is running on design flow rate or turndown flow; instead, it is impacted by the liquid level in the vessel, operating pressure, and operating temperature of the vessel at the moment fire happens. As because the liquid level contributes to the wetted area calculation and P & T play a critical role to know the time length when the relief valve will open, how much heat energy will be generated, and how long it will consume before the vessel get pressurized to the PSV set pressure. In this article, the author has explained the fundamental concept to calculate the PSV relieving temperature for fire case, and also enumerated the step by step guide to build the HYSYS steady state model.

Readers might be interested to know why this topic has been selected, and what is so special & important in calculation of PSV relieving temperature for fire case?

In PSV sizing calculation for fire case, as a Process Engineer, we use process simulator like AspenHYSYS to do the modeling and obtain the results for further design work. Many methods are being used by the process design engineers with different approach for simulation modeling to calculate the PSV relieving temperature for fire case. Author here has explained why the proposed method of simulation modeling is more realistic and accurate in comparison to other methods being used by the process engineers in design consultancies.

First of all, readers should know that both Pressure Safety Valve (PSV) or Pressure Relief Valve (PRV) are almost the same; the only difference is in opening mechanism, and the time of opening when the PSV or PRV set pressure hit due to some emergency or unexpected disturbance in the plant operation. PSV and PRV both act as the ultimate protection mechanical device for any equipment. PSV is designed to protect the equipment for burn-out conditions where all the primary protection by means of instrumentation or electrically controlled devices is burnt out during the fire case. Therefore, PSV is the very important device to protect the vessel in case of fire, where all other devices already stopped working due to the burn out of electrical and instruments facilities. As PSV plays such an important role in safeguarding the equipment on which it is mounted, therefore it should be designed with utmost care. The major parameters for designing a PSV is its orifice area calculation based on the maximum allowable peak mass flow from the PSV discharge to flare network during fire; and the second parameter is calculation of PSV relieving temperature for verifying the PSV outlet line material temperature design limits. This article also explains about the PSV relieving temperature calculation for fire case by using process simulator AspenHYSYS. It is assumed that process engineers working in the design industry might be doing HYSYS simulation and might be capable enough for calculating the PSV orifice area for the maximum allowable peak mass flow or PSV peak load.

Simulation & Modeling Executive Summary
To develop the HYSYS simulation model for determining the most accurate relieving temperature, we need to build HYSYS model in such a way that it represents the exact same situation which happens during fire, especially across the equipment on which PSV is mounted. During fire case, equipment is isolated by automatically closing inlet and outlet valves across a vessel. Now there is a trapped inventory in the vessel closed loop, which consists of some piping volume and the vessel volume. There may be two scenarios for calculating the inventory volume trapped in the equipment loop - normal liquid level operating case and the maximum liquid level operating case. The inlet flow rate does not get impacted by building a HYSYS simulation model no matter whether plant is running on design flow rate or on turndown flow. What matters is - liquid level in the vessel, operating pressure, and the operating temperature of the vessel at the very moment it catches fire. This is as because the liquid level contributes to the wetted area calculation, and P & T plays a very critical role to know the time length required for relief valve to open, the amount of heat energy to be generated, and also the time to be consumed before vessel get pressurize to the PSV set pressure.

Basis and Assumptions
1. There is a vessel on which PSV is mounted and the normal operating liquid level in the vessel is 40 percent. In real scenario, it can be any value depending on the process and its operation.
2. Volume of the vessel and that of the associated piping have already been calculated. The combined volume of vessel and associated piping is known as total volume of the vessel.
3. Although it is assumed that vessel's normal operating liquid level is 20m3 and the total vessel volume as per vessel dimension is 100 m3, in real case it will be different. Therefore it is to be kept accordingly.
4. It is assumed that readers are aware of the basics of AspenHYSYS viz. steady state simulation and modeling, defining the material stream, simulating it to get the fluid properties, and the model building components.

Simulation Methodology
This simulation is to be performed in steady state by using multiple adjusters.

1. First, it is to define the vessel inlet fluid streams in HYSYS at normal operating pressure and temperature, and to simulate it.
2. In HYSYS model, a two-phased separating vessel is to put to connect the inlet material stream to this vessel.
3. The liquid volume denotes the total liquid volume in the vessel, in addition to the one trapped in the associated piping. If the vessel inlet stream is of two-phased and the fluid trapped in the piping has both liquid phase as well as vapor phase, then the associated piping volume is to add accordingly based on the liquid volume fraction. Let’s say, in the vessel inlet, the stream is two-phased; it has 20 percent vapor fraction and 80 percent liquid fraction; and the associated calculated piping volume is 10 m3. Therefore the total liquid volume in the vessel is 10+20=30m3.
4. Next step is to define the vessel volume in m3; and it is also to manually adjust the liquid level percentage in the vessel to define the liquid volume 30 m3.
5. In the fifth step, the two outlet-streams are to be defined from this two -phased separating vessel. Top stream 1 is of the vapor outlet and the bottom stream 2 is of the liquid outlet.
6. Now, at this step, stream splitter is to be used to split stream one to 2 , 3, & 4; similarly stream 2 to be split to 5 & 6.
7. Further step is to adjust the mass flow rate of stream 4 to actual volume flow with the user defined level of 70m3. To run the adjuster, one needs to define some initial value of mass flow in the stream 3.
8. Now the mass flow rate of stream 5 is to adjust to its actual volume flow with an user defined level of 30m3. To run the adjuster, one needs to define some initial mass flow value in the stream 6.
9. A stream adder is to be used to add adjusted streams 4 and 5, and to define the adder outlet stream as 7.
10.Stream 7 here represents the actual liquid and gas volume condition in fire case when the vessel is isolated by closing the vessel inlet and outlet boundary valves.
11.During fire, this vessel gets heated up, however PSV will not pop up until and unless the pressure inside the vessel reaches to the PSV relieving pressure. It means, during the heating procedure because of fire, the mass inside the vessel will be constant till the vessel reach to the PSV relieving pressure; because the fluid mass will come out only when the PSV will open up at the relieving pressure, and thus will release the mass.
12.Now stream 8 is to be defined which has the same composition as stream number 7, pressure is to be defined as PSV relieving pressure, and the temperature as operating temperature.
13.Next is to adjust the temperature of stream 8 in such a way that mass density (mass/volume) of it remains the same as that of stream number 7.
14.Adjusted temperature of stream number 8 will be PSV relieving temperature for fire case.

Confirmation towards the Efficiency of the Model: This type of process simulation modeling for PSV relieving temperature calculation for the fire case is being used in many top design companies and well accepted by the operating companies like Shell,Petronace and Exxon mobile. Therefore, this is the first qualification of using this method; and the second qualification is the technical background which proves the practicality of this model to the fire case scenarios as compared to any other methods. In this model, by mass flow rates of gas and liquid streams coming out from the two phase separator is to be adjusted to the actual volume flow rate based on the actual vessel volume occupied by gas and liquid. It means if the adjusted gas and liquid streams are added to make the stream 7 in the model, then the actual volume flow of stream 7 will represent the actual condition of the vessel at steady state on which PSV is mounted. Stream 8 represents the actual situation when the vessel is isolated by closing the vessel boundary inlet and outlet valves. That’s why stream 8 is heated by adjusting the temperature keeping the mass density of the stream 8 same as the mass density of stream 7. As the volume of vessel is constant and the mass is also constant inside the vessel, till the inside pressure of the vessel increases to the PSV relieving pressure due to heating by fire, hence the mass density remains constant.