How to Design the Emergency Relief System

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DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

Each level represents an order of magnitude of risk reduction. Higher SIL level indicates higher is the safety level and lower probability that the system will fail to perform. The ideal time for SIL determination is

during the Front-End Engineering Design (FEED) and project definition stages, and typically as a supplement to the HAZOP. But SIL determination may also be used effectively during the plant’s life to determine if improvements are needed, and to provide guidance as to the form of the improvements.

SIL level can be identified based on the plant owner risk tolerance. Plant owner should determine the acceptable level of risk based on corporate strategy, legal requirement, capital and other factors. Selection of appropriate SIL level calls for a careful analysis.

Costs increase with higher SIL levels. It has been observed that typically the chemical industry has opted to go up to SIL level 2 based on the requirement.

Operational Risk Mitigation...

Shiva Engineering Services (SES) recently carried out ERS study for one of its polymer clients. As per the present status, the AOButyl Stearate Tank is currently protected by a rupture disk of 8”, set at 4-7psig @ 50°C. The purpose of this Emergency Relief System (ERS) design package was to examinethe adequacy of the existing installation with regards to overpressure protection. Also it included, if found inadequate, SES would design a relief system to protect the AO-Butyl Stearate Tank from unallowable overpressure.

Pressure rating of equipment: 70 psig @122°F Two-phase Vessel Dynamics was carried out in order to establish the relief rate. The relief rate was 159 lb/sec. Piping was analysed to find out maximum flow it could take. Then it was found that the piping was able to take flow rate more than 159 lb/sec. Maximum flow possible through piping was 180.5 lb/sec. Afterwards the complete study report was submitted to the client regarding the existing relief device and adequacy of piping to prevent the vessel from overpressure during external fire case.

Additional Information

Emergency Relief System

Designing the Emergency Relief: A Step–By–Step Instruction...

STEPS INCLUDED IN THE HAZOP METHODOLOGY

1. Collect data

2. Understand the subject

3. Prepare nodes and activities

4. Mark-up the drawings

5. Devise a list of appropriate keywords

6. Prepare table headings and an agenda

7. Select the team

8. Prepare a timetable

DATA TO BE COLLECTED BEFORE STARTING THE HAZOP STUDY (IN GENERAL)

1. Process description

2. P&I diagram

3. Process parameters

4. Material safety data sheet of

chemicals involved in the process

5. Interlock and logics

6. Start up and shutdown procedure

7. Emergency shutdown procedure

8. Prepare a timetable

SIL STUDY REQUIRES VARIOUS DATA (THESE WILL BE INCLUDED, BUT WILL NOT BE LIMITED TO)

1. All relevant studies (HAZOPs, QRAs, etc)

2. P&IDs

3. Cause and effect charts

4. Current maintenance and

shutdown details

5. Relevant operational information

...Through Emergency Relief System Design

This can be done both qualitatively and quantitatively and at varying levels of detail. To carry out qualitative risk assessments, ioMosaic’s ioFIRST software supports risk screening activities, while risk survey approach guides more detailed assessments. Fault-tree and event-tree analyses may be done to determine the frequency of potential incidents and evaluate risk for quantitative review. SuperChems – hazard assessment modelling software includes source term and vapour dispersion models.

* The author is the Delivery Head at Shiva Engineering Services, Vadodara.

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