Blocked Flow Overpressure
What is blocked flow overpressure?
This situation occurs when there is some type of blockage in the process flow path which causes any upstream pressure source (ex: pump or condenser) to continue building up pressure in the system. Without safeguards, all piping and process equipment between the upstream pressure source and the blockage will eventually be exposed to the maximum discharge pressure of the pressure source. Overpressure conditions exist when the process pressure exceeds the maximum rated pressure of any of the piping or process equipment.
What causes this condition to exist?
Below are some typical causes for blocked flow overpressure:
Inadvertent closure of a manual valve
Control valve failure
Hydrate formation
Freezing
Plugging of an orifice, strainer, or filter
What are the possible consequences of this scenario?
The consequences from this type of overpressure event can vary significantly and is dependent on various factors that contribute towards the overall severity of the situation.
Magnitude of the release
This is the ratio of the maximum source pressure to the maximum allowable working pressure (MAWP) of the piping or process equipment. A large release magnitude could result in process equipment exploding and affecting a larger vicinity and potentially leading to a fatality. A smaller release magnitude may simply damage a seal and cause a minor leak.
Properties of released substance
If the released substance is toxic, this could be very dangerous for the health and safety of nearby personnel or residential areas. Another consideration is if the substance is flammable and if there any nearby ignition sources such as static, a pump, or a vehicle.
Exposed environment
Is the loss of containment inside or outside of the building? Is it contained onsite or does it extend beyond the lease? Are there nearby environmentally sensitive areas that could be affected?
How can this be prevented?
This potential hazard can first be identified through a hazard and operability (HAZOP) study and then be further evaluated through a layer of protection analysis (LOPA) study to ensure there are adequate safeguards in place to mitigate the likelihood of this scenario from occurring.
Some important questions to ask and discuss with a PHA team are as follows:
What is the pressure source? What is the normal operating pressure and the maximum discharge pressure?
Where are the pressure spec breaks (if any) and what are the pressure ratings for each piece of equipment and connected piping?
How fast will the overpressure occur?
What safeguards are in place? (ex: high pressure shutdown or a pressure release valve)
Example of a blocked flow overpressure scenario:
