It recently came to my attention that several operating companies are opening parallel, 100% sized, relief valves to the process when the design was to leave one closed and one open to facilitate maintenance. This is apparently becoming a standard practice.
Normal sizing practice involves finding the highest relief capacity required and selecting a valve with at least that capacity. It is good practice not to size too large and 140% maximum is a good rule of thumb.
There are two scenarios where dual relief valves are installed. One involves two valves with staggered setpoints and a combined capacity of 100+%. This is not the subject of this Thread. The 2nd scenario is where two 100+% valves are installed and one is intended as a spare.
When parallel relief valves are installed in this manner and each is sized for 100+% of required capacity, the intent is to provide a spare that is closed to the process; this facilitates maintenance and testing.
It is very bad practice, (even dangerous practice), to open both of these valves to the process. In all fairness, this is done because it is perceived to be safer. In reality, it is not - it is less safe.
Without going into detail here, flare loading and pipe stress calculations are made based on P&ID's and the philosophy at the time of design. To open both valves to the process when this was not intended can result in a number of unwanted consequences to included:
- Transient Flare System overload - Capacity and Pressure
- Pipe Support Damage
- Relief pipe damage
- Damage to relief valves
- Less than required relief capacity *!*
- Liability: Plant Operating not in accordance with approved P&ID's *!*
Please see my paper on this subject @
or, see attached
v/r
jrlaw
Normal sizing practice involves finding the highest relief capacity required and selecting a valve with at least that capacity. It is good practice not to size too large and 140% maximum is a good rule of thumb.
There are two scenarios where dual relief valves are installed. One involves two valves with staggered setpoints and a combined capacity of 100+%. This is not the subject of this Thread. The 2nd scenario is where two 100+% valves are installed and one is intended as a spare.
When parallel relief valves are installed in this manner and each is sized for 100+% of required capacity, the intent is to provide a spare that is closed to the process; this facilitates maintenance and testing.
It is very bad practice, (even dangerous practice), to open both of these valves to the process. In all fairness, this is done because it is perceived to be safer. In reality, it is not - it is less safe.
Without going into detail here, flare loading and pipe stress calculations are made based on P&ID's and the philosophy at the time of design. To open both valves to the process when this was not intended can result in a number of unwanted consequences to included:
- Transient Flare System overload - Capacity and Pressure
- Pipe Support Damage
- Relief pipe damage
- Damage to relief valves
- Less than required relief capacity *!*
- Liability: Plant Operating not in accordance with approved P&ID's *!*
Please see my paper on this subject @
or, see attached
v/r
jrlaw
