Pressure Relief Devices (RV/RD/VB)

The functionality of relief valves can be categorized as:

• Primary:
  • To avoid over-pressurization of associated Equipment and Piping.
• Secondary:
  • To provide pressure relief capacity.
  • To separate the process from flare system (seat tightness).
  • To maintain pressure envelope integrity.
  • To lift to full travel when required and seal off when there is no demand (mechanical functionality).

Note: While the term PRV is sometimes used interchangeably with Pressure Safety Valve (PSV), there is a difference between the two. A PRV opens gradually in relation to the pressure, while a PSV is opened suddenly once the pressure hits a certain level in order to avoid over pressurization and a potential process safety incident. PSVs can be managed in IMS SIS (or SIFPro). 

Define Asset – Step 1

See Step 1.

Add a Component of Type "BODY" to the Pressure Relief Valve (PRV) Equipment. A Corrosion Loop always needs to be assigned to this Component. This can be the CL of the Equipment that the Relief Valve protects or all Relief Valves for a unit can be placed in one dedicated loop.

Create Component Type BODY.

 

Start RBI Analysis and Assign DMs to Component – Step 3

See Step 3.

For this Component type, IMS automatically assigns Demand Scenarios, rather than Degradation Mechanism (DM), to the Component when it is created. You can also add the Demand Scenarios manually.

Rather than a DM, Demand Scenarios is assigned to the Component.

When you start the RBI Analysis, you first need to define the Demand Scenarios:

• Click Demand scenario.

Click on Demand Scenario to define the Demand Scenarios.

 

• Click + and fil in the Demand Scenario name and notes.
• Click + to add more Demand Scenarios.

Determine Most Credible Failure Scenario – Step 4

See Step 4.

Define the Failure on Demand Scenarios.

For each Demand Scenario:

• Select the Criticality Assessment tab;
• Click on Demand Scenario;
• Highlight the Demand Scenario; and
• Click Scenario;
• Click Edit.
• Enter the Demand Scenario.
  Write the Failure on Demand Scenarios.

  
  

StF – Step 7

See Step 7.

Methodology

Demand Rate 

• Demand Rate reflects the number of times the PRV is required to lift and prevent over-pressurization.
• Where possible real lifting information should be used.
• The starting point to determine Demand Rates are the Relief Cases.
  • The safeguarding narrative or memorandum usually specifies as a minimum the 100% capacity relief cases.
  • A relief case generally starts with an “event” where the “event frequency” determines the Demand Rate.

Relief Cases are the scenarios that may cause over-pressurization of a system and affect the relief and flare sizing. Relief Cases are generally divided into three types (see the table below).

Relief Cases:

Note: A well written safeguarding narrative/memorandum specifies all the Relief Cases and generally, the event frequencies for those cases determine the Demand Rate. For example, “heat exchanger tube rupture” scores in the “>20yr” category.

Likelihood of Failure

When determining the Likelihood of Failure, one should focus on the likelihood of the three Failure Modes causing a Relief Valve to “Fail to Danger” on demand (see the table below).

Failure Modes:

Note: For pilot operated PRV’s it is suggested to use a minimum likelihood of “Low” because this type of PRV is more prone to plugging and/or fouling, due to the presence of small-bore tubing.

Confidence

The Confidence Rating reflects the confidence that the RBI team has in the Likelihood of Failure and depends on the stability of the process and the results of inspection/testing. The Confidence Rating is done for a specific PRV.

Confidence can lower the overall StF by one or two levels. Do not take credit for vents to safe location, as fouling, birds, and water accumulation cause issues. Cool service is valid if no temperature correction for test pressure is required (generally 100°C/212°F).

PRV Confidence Assessment Guidance:

Overall StF

IMS uses the Demand Rate and Likelihood of Failure to lookup the overall StF (see table below) for each Demand Scenario. The Confidence rating is then used to correct the StF based on “proven reliable service” of the PRV.

StF lookup table for PRVs:

Software

Do an StF for each Demand Scenario:

• Select the Criticality Assessment tab;
• Click on Demand Scenario;
• Highlight the Demand Scenario; and
• Click PRV StF to start the StF assessment.

Select a Demand Scenario and click PRV StF to start the assessment.

 

• Select the Demand Rate (i.e., how often there is a demand).
• Click Edit.
• Select the Likelihood of Failure (to open on actual demand).
• Answer the 3 Question (making use of the information buttons) to determine the Confidence.
• Click Close.

Repeat to create more Demand Scenarios.

Consequence – Step 8

See Step 8.

Methodology

Remember to first write down the failure scenario (see step 4 above). Not all failures have consequences.

For External Fires:

• Only assess incremental damage when the Relief Case is “external fire”; and
• Take Boiling Liquid Expanding Vapour Explosion (BLEVE) and steam explosions into account.

The table below summarizes the significance and potential Consequence due to pressure accumulation. Accumulation is measured as % Over Maximum Allowable Working Pressure (MAWP).

Potential Consequence due to Pressure Accumulation (% Over MAWP):

Software

Do a Consequence of Failure Assessment for each Demand Scenario:

• Select the Criticality Assessment tab;
• Click on Demand Scenario;
• Highlight the Demand Scenario; and
• Click Asset, People, Environment, and Community, to assess each of these consequences.

Click Asset, People, Environment, and Community, to assess each of these consequences.

Note: Consequence of Failure on Demand is NOT identical to the RBI Consequence for the Equipment that the PRV protects.

Confidence – Step 10

See Step 10.

The Confidence is evaluated in the StF section (see Step 7 above).

IS and MII – Step 11 & Step 12

See Step 11 and Step 12.

The highest Criticality of all Demand Scenarios becomes the overall Criticality. IMS uses a lookup table to determine the MII from this overall Criticality. For Criticality class E a redesign is advised.

MII lookup table for PRVs:

Assign to Inspection Schedule – Step 12

See Step 12.

Include the completed PRV RBI assessment into the scope of an Inspection Schedule that represents relief valve recertification.

Specify activities during recertification, e.g.,

• Visual
• As received pop test
• Disassembly/reassembly & visual
• Set Pressure
• Pop test