Pressure Relief Valve:Single Phase Relief

Author: Vikram Sharma Date: 12th March 2017

Table of Contents Introduction Type of Pressure Relief Valves Concept of backpressure Pressure level settings Calculation methodology Summary References

Introduction Pressure relief valves a.k.a pressure relief devices

(PRD) Primary function → protect an equipment frm.

overpressure that may lead to catastrophic incident.

Common → PRDs safety valve, PRV, safety relief valve, PORV & rupture disk

Focus on three types of PRVs: Conventional PRV Balanced-bellows PRV and Pilot Operated PRV

Calculation as per API Std. 520 Part 1 9th Ed. (2013)

Type of Pressure Relief Valves Types of PRV:

Conventional PRV; Balanced-Bellows PRV; and Pilot Operated PRV

Conventional PRV: Used when the built-up backpressure should not

exceed 10% of the set pressure at 10% allowable overpressure.

Higher allowable overpressure of more than 10% may allow a higher max. allowable built-up backpressure provided the built-up backpressure does not exceed the allowable overpressure.

Type of Pressure Relief Valves (cont’d) Balanced-bellows PRV:

Used when the built-up backpressure (superimposed + built-up) is too high for conventional PRV.

Used when the superimposed backpressure varies significantly in comparison to the set pressure

Used when the total backpressure (superimposed + built-up) does not exceed approx. 50% of the set pressure

Pilot Operated PRV: Valve lift is not affected by backpressure

Type of Pressure Relief Valves (cont’d)

Concept of Backpressure Backpressure consist of two parts that are:

Superimposed backpressure Built-up backpressure

Superimposed backpressure: Pressure originating frm. other sources when the PRV

is in READY MODE. Two parts that are variable and constant

Variable one or more PRVs discharging into a common header. Each PRV may have different backpressure at each moment @

each relief cycle Balanced & Pilot → used as backpressures vary significantly

under any operation condition Direct impact on the set pressure.

Concept of Backpressure (cont’d) Superimposed backpressure (cont’d):

Two parts that are variable and constant (cont’d) Constant

Occurs when the outlet of a PRV is connected to a static pressure source which does not change significantly under any operational condition.

Actual set pressure is defined as the sum of bench set pressure & backpressureConstant Variable

Concept of Backpressure (cont’d) Built-up backpressure:

Occurs when the PRV is in OPEN MODE and flowing due to the following reasons: Rate of fluid flow through the PRV; Size and configuration of the PRV discharge piping; and Other source of pressure acting into the discharge

header Affected by the friction and pressure drop through

the discharge piping. Built-up backpressure is always variable

Concept of Backpressure (cont’d)

Pressure Level settings Set Pressure

Pressure (inlet gauge pressure) at which the relief device set to open under service conditions

Accumulation Expressed as percentage of MAWP Defined as the pressure increased above the MAWP

Overpressure Pressure increase over the PRV set pressure Expressed in pressure units or percentage of set pressure

MAWP Maximum Allowable Working Pressure It’s a term related to the construction of a vessel or item

to be protected

Pressure Level settings (cont’d) MAWP (cont’d)

Defined as the max. allowable pressure at the top of a completed vessel in its normal operating position and at a designated temperature.

Pressure Level settings (cont’d) Confusion between accumulation & overpressure?

Overpressure is referenced to the set pressure which is a property of a relief valve.

Accumulation is related to MAWP which is a property of a vessel or item to be protected.

Confusion between MAWP & Design Pressure? MAWP → defined as the max. allowable pressure at the

top of a completed vessel in its normal operating position and at a designated temperature.

Design pressure → pressure with a margin above the most severe pressure expected during normal operation at a coincident temp.

MAWP is normally higher than the design pressure (API 520)

Pressure Level settings (cont’d) MAWP or Design Pressure for PRV sizing?

During design stage where MAWP is unavailable, designer is to rely on some basis for calc. → design pressure (Para 3.16 API 520 Part 1 9th Ed. (2013))

MAWP is a property assigned by the fabricator of the vessel

ff

MAWP is normally higher than Design Pressure

Set Pressure is also the Set Point of PRV & shall not exceed the MAWP

Pressure Level settings (cont’d) Max. accumulation & set pressure of a relief

valve is further divided by its configuration and relief case category. Relief case – Fire or Non-fire case Configuration – Single or multiple device

installations

Calculation Methodology Simplified P&ID of Fuel Gas (FG) Knock Out

(KO) Drum

(Source: MOHIB, 2016)

Calculation Methodology (cont’d) Important facts

Assume the KO drum is at design stage. Set Pressure (SP) = Design Pressure of the drum Backpressure ≤ 50% of the SP → Balanced-Bellows PRV Design data:

Gas density (ρG): 4.1 kg/m3 Ratio of specific heats, (Cp/Cv) = k: 1.55 Compressibility factor (Z) = 0.95 Molecular Weight of FG (MW): 20.0g/gmol Relieving Temp. (T): 20°C Set Pressure (SP): 4.5 barg Accumulation: 10% Backpressure @ relief valve discharge: 2.1 barg

Calculation Methodology (cont’d) Check if the PRV conforms to critical or sub-critical

flow condition What is critical flow condition?

Expansion process seen when a compressible fluid (gas) flows across a nozzle at constant U/S condition

Results to increased gas vel. & specific volume with decreasing D/S pressure

At constant U/S condition, the mass flow ↑ to a point where further ↓ in D/S pressure will not see ↑ in gas flow: Critical flow rate

Calculation Methodology (cont’d) Check if the PRV conforms to critical or sub-critical

flow condition What is critical flow condition? (cont’d) Determine the Critical flow pressure (Pcf)

Require info. : upstream relieving pressure & Cp/Cv @ ideal condition @ relieving temp.

P1 is a f(SP, Allowable overpressure, Patm)

Calculation Methodology (cont’d) Check if the PRV conforms to critical or sub-critical

flow condition Determine the Critical flow pressure (Pcf) (cont’d)

Critical flow: Downstream pressure (P2) ≤ Pcf

Sub-critical flow: Downstream pressure (P2) or backpressure Pcf

Backpressure > Pcf → SUB-CRITICAL FLOW

Calculation Methodology (cont’d) Check if the PRV conforms to critical or sub-critical flow

condition Calculate the relief discharge area (A)

PRV is w/o a rupture disk; Kd = 0.975 for PRV installed with / w/o rupture disk, Kd = 0.62 when PRV is not installed

Kc = 1.0 for PRV is not installed with rupture disk, Kc = 0.9 for PRV installed in combination with rupture disk

Calculation Methodology (cont’d) Calculate the relief discharge area (A) (cont’d)

Kc = 1.0 for PRV is not installed with rupture disk, Kc = 0.9 for PRV installed in combination with rupture disk (cont’d)

Sizing eq. for PRD for vap. & gas service based on the following assumptions: Pressure-specific volume relationship conforms along the

isentropic path Assumption may not be valid for the following conditions:

At very high pressures; and Gas or vapours approaching the thermodynamic critical locus

Compressibility factor, Z, provides an indication whether the gas or vap. may be in the above conditions, i.e. Z < 0.8 or Z > 1.1. Refer to Annex B of API Std. 520 Part 1 9th Ed. (2013)

Calculation Methodology (cont’d) Calculate the relief discharge area (A) (cont’d)

Summary

References "Pressure Relief Valve Sizing Calculations". (2017). Pressure Relief Valve Sizing Calculations – Subcritical Gas

Flow Service. Retrieved January 12, 2017, from Engcyclopedia: http://www.enggcyclopedia.com/2011/11/pressure-relief-valve-sizing-calculations-subcritical-gas-flow/

API. (2013, December). API Standard 520 Part 1. Sizing, Selection, and Installation of Pressure-relieving Devices, 9th. Washington, D.C: American Petroleum Institute.

Coker, A. K. (2006). Process Safety and Pressure-Relieving Devices. In Applied Process Design for Chemicals and Petrochemical Plants (4th ed., pp. 575-578). Oxford, 1: Gulf Professional Publishing.

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Hellemans, M. (2009). Terminology. In The Safety Relief Valve handbook: Design and Use of Process Safety Valves to ASME and International Codes and Standards (1st ed., p. 44). Burlington: Butterworth-Heinemann.

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