Risk Analysis for Pressure Relief Devices – Managing Plant Safety and Reliability

Risk Analysis for Pressure relief devices – Managing Plant Safety and Reliability

Pressure relief devices are one of the most critical assets for plant risk and safety, therefore it is essential to be able to ensure PSV's operation, reliability and availability through the implementation of risk mitigation or prevention actions, either, corrective, preventive or predictive; these actions will allow plant owners to manage the risk associated with the device operation (or failure). This paper is intended to show a risk assessment using Weibull analysis focused on the identification for risk reduction opportunities and the selection for the best chance and dates to implement risk mitigation actions. Risk assessment methodology is based on API RP 581, Risk Based Technology.

Historically, maintenance planning for a PSV was carried out, based on pressure tests and engineering judgment; a pressure (POP) test is usually conducted to evaluate device's availability and functional condition and its frequency is linked to the protected equipment's maintenance program, regardless whatever the failure mode. Nevertheless, now a days, more quantitative and rigorous methodologies are required not just for maintenance planning but for developing a risk management strategy.

The methodology is a risk assessment. We evaluate the probability of failure (POF) times the consequence of failure (COF), for both, a valve failure and a protected equipment failure. Two failure modes are evaluated: fail to open on demand (FOD) and leak (LEAK). POF is calculated analyzing three main issues:

1.      Historical inspection records and time in service, using a two parameters Weibull distribution, as follows:

                                                                 (1)

2.      Probability of failure from the protected assets,

                                                                             (2)

3.      Probability of failure from process over pressure scenarios.

                                                             (3)

Consequences of failure (FOD, LEAK) are evaluated as a function of consequence area and as a function of its economic impact, for a valve failure and for a protected equipment failure.

FOD consequences are evaluated for four leak scenarios (1/4”, 1”, 4” and catastrophic rupture) for each protected equipment and using an overpressure as the discharge pressure. This overpressure is a higher pressure than the normal operating conditions. In this approach impact zones are used to evaluate fire, explosion and toxicity effects, in the same way we do it for a traditional quantitative consequence analysis. Meanwhile, for the leak failure mode, consequences are evaluated using a financial approach, no impact area is considered. Consequences categories for the leak mode are as follows:

·         Loss of production costs

·         Maintenance cost (repair costs)

·         Product costs

The leak mode includes both, a small leak and a moderate leak. Moderate leak means valve stuck open, while small leak means leakage past valve (No seal).

Therefore, the global risk is calculated using the two risks approaches,

                                           (4)

Risk calculation involves a straightforward review of the process operating conditions using a layer of protection analysis (LOPA), it also requires analyzing the process conditions and initiating events than can challenge or demand the PSV and the layers affecting the initiating event frequency. The methodology also involves a process for evaluate as many overpressure scenarios as possible; this scenarios will also challenge the pressure relief device.

This risk analysis methodology has the following benefits:

a)      Perform a What if…? Scenarios analysis, and evaluate risk mitigation actions

b)      Update analysis modeling, using new data

c)      Understanding risk drivers

d)     Understanding event sequence for risk contributors

e)      Set up a risk management strategy

The risk assessment was performed for 100 PSV, located on several offshore facilities. Results show risk contribution for every failure mode, risk ranking for the whole set of PSVs, new maintenance dates as well as reliability and availability estimations. The use of this approach is very useful, because we are able to deal with large data samples and also fits well with small data samples. Results can be expressed using failure mode information or time to failure information.