Rupture Disk Valve

Rupture disc, also known as bursting disc/pressure safety disc/burst disc is a pressure relieving device used in Chemical, Pharmaceutical, Aerospace, Aviation, Defence, Railroad, Nuclear, Food Processing, Petrochemical industries. The main purpose of the disc is to release excess pressure and prevent any structural/functional damage to the instruments/machines.

Functionality:

A Rupture disc is fitted inside the pressure pipes/instruments & is designed to rupture/burst/tear at a pre-defined pressure. It can be calibrated for a pre-determined pressure (positive) or vacuum conditions.
When the disc is designed for vacuum conditions, the atmospheric pressure would push this disc into the system. Hence, a support device is used to prevent the atmospheric pressure from pushing the disc into the system. Thus, when the pressure rises, the disc ruptures and releases excess pressure thereby protecting the system from damage.

Uses:

1. As the name suggests, the rupture disc ruptures and thereby prevents a catastrophic damage that would otherwise be done to the structures/machines

2. The disc forms the weakest side of the system and prevents the other parts of the system from suffering pressure damage

3. A key feature of the rupture disc is that it is placed well away from the heavily traversed area of the factory. Thus, harmful chemicals/fumes are released away from the workers and can also be connected to discharge pipes which safely guide the exhaust fumes/fluids

4. Minimum maintenance costs

Limitations:

One of the biggest limitations of a rupture disc is that once burst, the entire system has to be shut down to replace the disc. The downtime on shutting down the entire system and then rebooting it can cost some heavy losses in price sensitive & highly competitive markets.
To overcome this disadvantage, designers were sent back to the drawing board for designing an improvised version of the rupture disc.

Relief Valve:

A relief valve serves the same purpose as the rupture disc but has the added advantage of not shutting down the system. The inlet is threaded and connected to the system. The outlet is threaded and connected to the exhausts. The system is embedded with a spring which controls the release of pressure.

The Rupture Disc was implemented by PANORAMA ENGINEERS at VERTEX PHARMACEUTICALS.

HAZOP Guidelines

HAZOP uses a brainstorming approach around a series of guide words designed to qualitatively identify possible deviations from normal operation and their possible impacts. Responsibilities are assigned to investigate possible solutions for each problem found.

Guidance is given on study procedure and prerequisites for an effective HAZOP, including team selection, information requirements and record keeping.

To be effective, a HAZOP study must be systematic, detailed and conducted by a balanced team with an experienced leadership.

Effective HAZOP strategy:

The effectiveness of a HAZOP will depend on:

  • the accuracy of information (including P&IDs) available to the team — information should be complete and up-to-date
  • the skills and insights of the team members
  • how well the team is able to use the systematic method as an aid to identifying deviations
  • the maintaining of a sense of proportion in assessing the seriousness of a hazard and the expenditure of resources in reducing its likelihood
  •  the competence of the chairperson in ensuring the study team rigorously follows sound procedures.

Key elements of a HAZOP are:

  • HAZOP team
  • full description of process
  • relevant guide words
  • conditions conducive to brainstorming
  • recording of meeting
  • follow up plan

HAZOP Worksheets:
The HAZOP work-sheets may be different depending on the scope of the study.
Generally the following entries (columns) are included:

  • Ref. no.
  • Guide-word
  • Deviation
  • Possible causes
  • Consequences
  • Safeguards
  • Actions required (or, recommendations)
  • Actions allocated to (follow-up responsibility)

HAZOP Pre-requisites:

As a basis for the HAZOP study the following information should be available:

  • Process flow diagrams
  • Piping and instrumentation diagrams (P&IDs)
  • Layout diagrams
  • Material safety data sheets
  • Provisional operating instructions
  • Heat and material balances
  • Equipment data sheets Start-up and emergency shut-down procedures

HAZOP Procedure:

Though there are no fixed approaches, following is a typical HAZOP procedure:

  1. Divide the system into sections (i.e., reactor, storage)
  2. Choose a study node (i.e., line, vessel, pump, operating instruction)
  3. Describe the design intent
  4. Select a process parameter
  5. Apply a guide-word
  6. Determine cause(s)
  7. Evaluate consequences/problems
  8. Recommend action: What? When? Who?
  9. Record information
  10. Repeat procedure (from step 2)

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HAZOP Modes of Operation:

The following modes of plant operation should be considered for each node:

  • Normal operation
  • Reduced throughput operation
  • Routine start-up
  • Routine shutdown
  • Emergency shutdown
  • Commissioning
  • Special operating modes

A sample HAZOP process worksheet is illustrated in the below figure:

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HAZOP Outline:

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The key point here is that a HAZOP study must promote freethinking by the team members around each issue so that most possible problems can be identified. At the same time, the HAZOP company must impose enough discipline to keep the study moving along without wasting time on issues that are of no consequence.