How to Select the right Product to meet pharmaceutical Pressure Equipment Standards
There are numerous production and process requirements which exist and apply specifically for particular industries. This is certainly the case in the pharmaceutical industry, where installations and the materials used are required to meet specific pharmaceutical standards. Additionally, they also have to meet the general process requirements and norms, one of which is the PED or the Pressure Equipment Directive.
Understanding the Pressure Equipment Directive
The PED applies to the design, manufacture and conformity assessment of stationary pressure equipment with a maximum allowable pressure greater than 0.5 bar. To prevent the pressure from exceeding the design pressure of the installation, a pressure safety device is needed. Every installation needs a device that opens itself from the existing process pressure, when the maximum process pressure is reached.
Given the aseptic nature of the pharmaceutical processes, a pressure device that hermetically seals the process is preferred, in order to prevent unwanted contaminants from entering or exiting. There are two options (or a combination of both) which are able to provide this:
Option 1: Pressure Relief Valve
The working principle of a pressure relief valve (PRV) is simple; a valve is closed by a spring and opens once the pressure reaches an undesirable level. Because of the simple installation and operation, a PRV is the most logical choice. However, there are a few issues with PRVs which are not generally well-known, but very important where the pharmaceutical industry is concerned.
- The PRV is never 100% closed, even when it’s brand new. The spring closes the valve, but there is always a certain leak rate – specified in API 527 and EN ISO 4126-1.
- It’s difficult to see whether the PRV has been open or not, as there is no indication given. Detection systems with proximity switches exist, but these only give a signal at a significant opening. This means that not all openings are reported, meaning there is a risk of insidious leaks.
- Once the process pressure reaches the operating pressure the leak rate increases; again without indication.
- Also the cleanability of PRVs is an issue. There are models which can be installed close to the tank wall, but these are very expensive. The standard models have an installation distance (L/D ratio) that is far beyond the cleanability norm and can therefore be a source of infection.
- According to the PED, a PRV need to be tested every 2 years, as well as each time it has been activated. Meaning a lot of downtime and money.
Conclusion: a PRV is a simple and good pressure safety device, but it has many hidden disadvantages in aseptic processes.
Option 2: Rupture Disc
A rupture disc is a metal plate with a precisely defined thickness, which results in a very accurately break point within the 95% operating ratio. In addition to this advantage, rupture discs provide many other benefits:
- No leakage – a rupture disc is completely closed
- Installation directly on a tank, so no L/D and a high cleanability
- A rupture indicator can be added which provides a signal once the rupture disc opens
- Operating ratio of 95%
- KR-value – the opening of a rupture disc is larger than the opening of a PRV with the same connection size
- A rupture disc is much cheaper – if you want a device with the same KR-value, then a rupture disc is six to 12 times cheaper than a PRV
Also importantly, a rupture disc doesn’t have to be replaced or tested every two years. It can stay on a process as long as it needs to, as long it hasn’t been ruptured. Plus, when the disc bursts, it can be directly replaced with a stock item. Rupture discs can be kept on the shelf for more than 10 years – it will not change their performance level.
The only disadvantage of a rupture disc is that it doesn’t close again after bursting.
Combining Pressure Relief Valves and Rupture Discs
Pressure relief valves and rupture discs, when combined, create a symbiotic relationship that hermetically seals your pressure device, protect an expensive pressure relief valve from costly replacements with an affordable, pressure relieving rupture disc. By placing a PRV together with a rupture disc, you get the best of both worlds.