PRV volume sizing of the shell and tube heat exchanger

[*phillip*]

We operate a shell and tube heat exchanger, where steam flows on the shell side and feedwater flows on the tube side. The feedwater pressure is higher than the steam pressure. A pressure control valve (PCV) is installed to prevent overpressure on the shell side, and in the event of continued overpressure, the entire system is designed to trip. Additionally, the PCV is interlocked not to open unless feedwater is flowing into the heat exchanger.

There is also a pressure relief valve (PRV) installed on the shell side, which is sized according to the HEI Code. Given that the system is interlocked to prevent steam overpressure, is this design approach(PRV sizing accordance with HEI code) commonly accepted in the industry?

 

[goutam_freelance]

In a normal thermal power plant, there is no PCV, and the heating steam sources are uncontrolled extraction points from the steam turbine. On the other hand, for PRV sizing, we used to follow the HEI code.

In your case, you are using a controlled external steam source. As per ASME code, in spite of other protections, you should have a safety valve downstream of the PCV sized to pass the steam flow at shell design pressure plus an offset pressure in case the PCV fails open.

But for the shell side relief valve as per HEI, you should design the water relieving capacity based on the condition when there is one or multiple tube ruptures.

The sizing philosophies are different, and yes, you should have a shell side relief valve as per HEI, too.

However, the above refers to the power plant industry, and I do not know whether the same will be applicable to your industry. However, it is safe to follow ASME and HEI.

 

[pierreick]

Hi,
You may want to share with data about your process (heat exchanger), Flow rates, pressures and temperatures.
What are the consequences if water is contaminating the steam flow?
Pierre

 

[*phillip*]

Thanks for the reply. I am referring to a thermal power plant. The steam turbine consists of HP/IP/LP sections, and a heater is connected downstream of the IP turbine. PCV is installed between the IP turbine and the heater to regulate the steam flow to the heater. Even if the PCV is fully open, the steam pressure does not exceed the heater’s design pressure.

During plant startup, before the turbine begins operation, or the plant is tripped, high-pressure steam is temporarily dumped into the heater through a pressure reducing station (PRS). However, even when the PRS is 100% open, the heater’s design pressure is not exceeded since PRS structurally throttles.

In rare situations where the feedwater supply to the heater is insufficient, the pressure inside the heater could gradually rise. In such a case, the PRS is interlocked to be closed if the pressure becomes too high. If PRS is not closed due to some reasons such as leak and so if the pressure is still high, then the heat source like a gas boiler system is interlocked to be tripped, and the high-pressure steam is vented through the start-up vent in the line.

Given this protection logic, the pressure relief valve (PRV) for the heater is currently sized in accordance with the HEI Code. And some say it is an engineering practice.

 

[goutam_freelance]

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It is clear from your post that the steam side steam pressure will not exceed the safety limits of the heater. So the steam side safety valve is not required.

Even then a shell side relief will be required. Consider the case when some of the tubes are ruptured. The tube side pressure will be much higher than the shell side pressure and this potentially can create a dangerously high pressure in the shell side. So the shell side including the PRS require protection. The PRV will do this as per HEI

One observation though, the PRS is generally not used in high efficiency thermal power plants where heater supply steam pressure floats with the bleed presdure. You definitely have some reason for this, but this introduces inefficiency in the system.