For those of you who are regular readers you may have noticed that I have been a bit sporadic with my posts of late due to work committments. Hopefully though I should be able to get back on schedule with a post a month.
For this one I thought I would take a small amount of time to talk about relief systems; more specifically relief valves and their sizing. I remember when I was at university that I understood the concept of pressure relief valves (PRVs) and why they were employed (as we don't engineer inherently safe designs) but that was really the limit of my understanding. Hopefully this small post will give you help if you have to do any relief valve sizings in design projects etc.
The main industry standard for PRV sizing is API 520 Part 1. This standard gives a really good overview about pressure relieving devices and explains how you size the orifice inside the PRV controlling the release of overpressure. It also gives guidance on the type of relief scenarios you need to consider, i.e. blocked outlet, fire impingement on a vessel, etc.
Lets take an example of a reactor with a design pressure of 200 barg. The PRV may be set to 200 barg so that in an instance where the maximum allowable operating pressure is exceeded the pressure in the vessel the design case is not exceeded.
The standard guides you through how to determine backpressures on the relief valve (e.g. hydraulic loads on the outlet of the valves caused by other flows or frictional losses in the downstream vent system) and other overpressures which lead you to a set of relieving conditions for the device. These conditions in addition to the reliving fluid properties allow you to calculate the required effective discharge area of the PRV.
This area is then translated into an orifice size from which you can determine the valve body size from a source such as the GPSA Databook. Following this method allows you to obtain all of the necessary process design information for a PRV datasheet. If you want to take it one step further, API 520 Part 2 describes the method for sizing the inlet and outlet pipework to the PRV. The sizes are a function of pressure loss and velocity constraints.
If anyone would like more information feel free to add a comment to the post. Until next time.....
