OVERPRESSURE PROTECTION

PHILOSOPHY

M. Surya Abadi Ginting

November 30, 2013

Short Course

Untirta, Serang - Banten

Course Content

1. Introduction

2. Definitions

3. Causes of overpressure

4. Types and applications of pressure safety valves

5. Limitation of relieving pressure

6. How does it work

7. Avoid Chattering

8. Sizing of pressure safety valve for gas or vapor service

9. Sizing of pressure safety valve for liquid service

10.Sizing for vaporizing liquid

Introduction

All pressure vessel shall be adequately protected against overpressure. It can be done by installing Pressure Relieving Devices on it such as pressure safety valve or rupture disc.

The function of relief devices is to:

1. Prevent overpressure

2. Remove excess fluid safely

3. Protect equipment, piping and people

4. Prevent loss of material

Relief devices are mandated by industry codes and standards, national, state, client and corporate requirements.

1. Pressure Relief Valve (PRV) : a pressure relief device (PRD) designed to open and relieve excess pressure and to reclose and prevent the further flow of fluid after normal conditions have been restored (API 520, 2000).

2. A Safety Valve is a spring loaded pressure relief valve actuated by the static pressure upstream of the valve and characterized by rapid opening or pop action (API 520, 2000).

3. Set Pressure is the inlet gauge pressure at which the PRD is set to open under service conditions (API 520, 2000).

4. Blowdown is the difference between the set pressure and the closing pressure of a PRV (API 520, 2000).

Definitions

Definitions

5. Back Pressure : the pressure that exists at the outlet of a

Pressure Relief Device (PRD) as a result of the pressure in

the discharge system

6. It could be either constant or variable. It is the sum of

superimposed and built-up back pressures

7. Superimposed back pressure is the static pressure that

exists at the outlet of a PRD at the time the device is

required to operate

8. Built up back pressure is the increase in pressure in the

discharge header that develops as a result of flow after

the PRD or devices open

9. Maximum Allowable Working Pressure : the maximum

gauge pressure permissible at the top of a completed

vessel in its normal operating position at the designated

coincident temperature specified for that pressure

10. Accumulation is the pressure increase over the MAWP of

the vessel allowed during discharge through the pressure

relief device, expressed in pressure units or as a

percentage of MAWP or design pressure

11. Overpressure is the pressure increase over the set pressure

of the relieving device allowed to achieve rated flow. It is

the same as accumulation when the relieving device is set

to open at MAWP of the vessel

Definitions

Causes of Overpressure

1. External fire

2. Blocked outlet

3. Automatic control failure

4. Tube rupture

5. Thermal expansion of blocked in fluid

6. Reflux failure

7. Utility failure such as cooling water, instrument air, etc

8. Abnormal heat or vapor input

9. Process changes/chemical reactions

External Fire

1. 21% overpressure of equipment that exposed to fire (ASME Sec VIII)

2. Pressure relief protection of piping from fire exposure is not required (KBR)

3. Equal to or less than 25 ft above the source of the flame needs to be considered (API 2000)

4. If many equipment are protected by a single PSV for fire case, interconnecting piping that expose to fire should be included in the wetted surface area

5. Air fin coolers that elevated above 25 ft do not require relief protection

External Fire, Pool Fire

Blocked outlet Other Scenario

Blocked Outlet

It occurs due to a number of reasons such as:

1. Instrument failure

2. Mechanical failure

3. Inadvertent valve closing by operator

4. Utility failure

The required load capacity due to blocked outlet can be

determined based on mass balance in the system. The system is

shown as below. What is the required load capacity?

Blocked Outlet

V-5

Automatic Control Failure, Gas Blowby

The automatic controller can fail because of :

1. Instrument air failure

2. Mechanical malfunction of control valve

3. Improper manual operation by operator

4. Plugging

5. Fail open or fail close must be evaluated

Credit can be taken for interlocks on ESD system only for the

flare header load and not for individual relief valve sizing.

Please refer to the following scheme, based on mass balance,

what is the load capacity once the CV is stuck open?

Gas Blowby

Stuck opening

Tube Rupture

1. Relief protection due to tube rupture is not required if the

design pressure of low pressure side is equal to or higher

than 2/3 of the design pressure of high pressure side. The

2/3 is generated from the test pressure design criteria. The

test pressure can be either1.3 or 1.5 of design pressure

depending on the process fluid (ASTM B.31.3)

2. Vapor flow, total relief load is W1 + W2 or 2W2

whichever is greater (see the Example)

3. Liquid flow, total relief load is W = 2x1343xd2(P)1/2

4. Tube rupture may occur at the tube or at the back side of

tubesheet

Thermal Expansion

1. Thermal expansion of liquid filled equipment and piping is required if they can be blocked in and subsequently heated by :

a) Solar radiation

b) Hot side of heat exchanger

c) Heat Tracing

2. Applicable for long section of piping and cold side of shell and tube exchangers

3. 10% overpressure is allowed for vessels and 33% for piping

4. 3/4” x 1” RVs is usually applied for thermal protection on piping and calculation is not required

Types of Pressure Safety Valve

1. Conventional Back Pressure < 10% Set Pressure

its outlet is usually vented to atmosphere due to non-toxic,

its operational characteristics are back pressure dependent

2. Balanced Back Pressure < 35% Set Pressure

Its bellow could minimize the effect of the back pressure

Protects spring from corrosion

3. Pilot It is not affected by built-up back

pressure

Conventional

Balanced

To minimize the effect

of back pressure

Pilot

Limitation of Relieving Pressure

Relieving pressure shall not exceed:

1) 3% for fired and unfired steam boiler

2) 10% for vessel equipped with a PRD

3) 16% for vessel equipped with multiple PRD

4) 21% for fire contingency, either one or multiple

How does it work?

How does it work?

Vapor or Gas Service

Its operation is based

on a force balance

How does it work?

Vapor or Gas Service

How does it work?

Vapor or Gas Service

Non-Vented Bonnet or Vented to ATM

Increase in backpressure will increase PSV opening pressure (higher than set pressure).

Set pressure shall be reduced by adjusting spring setting (reduce Fs) or vice versa

Lift of Disk

Avoid Chattering

Chattering is the resulting of:

1. Excessive Inlet Pressure Drop

2. Excessive built-up Back Pressure

3. Oversize Valve

4. Valve handles at high different rate

Process engineers or instrument engineers need to know the fundamental of pressure safety valve in order to be able to size a PSV properly based on API RP

520 and 521 to avoid the chattering.

Sizing PSV for Gas/Vapor Service

Critical gas flow behavior : either critical or subcritical

Critical if pressure downstream of the nozzle (back

pressure) critical flow pressure (Pcf)

Subcritical if the pressure downstream of the nozzle Pcf

Critical flow pressure ratio is an isentropic coefficient

dependent as shown below:

If the set pressure of the PSV is less than 30 psig, ASME

Section VIII specifies a max. allowable overpressure of 3 psi

Critical Flow

Under critical flow condition, effective discharge area of

pressure relief devices in gas or vapor service is determined

based on the following formula (SI):

while for steam service using (SI)

A = required effective discharge area, mm2

W = required mass flow through the devices, kg/hr

C = 315 if k is unknown, the coefficient is determined from

k = Cp/Cv

Kd = effective coefficient of discharge (either 0.975 or 0.62)

P1 = upstream relieving pressure in absolute

Kb = correction factor due to back pressure only for bellow

Kc = combination correction factor, (with 0.9 or without 1.0)

T = relieving temperature, R

Z = compressibility factor

Critical Flow

Critical Flow

M = molecular weight

V = required flow through the devices, Nm3/min at std

G = specific gravity at std

KN = correction factor for Napier equation

= 1.0 if P1 1500 psia, if P1 > 1500 psia, KN equals to

KSH = superheat steam correction factor

= 1.0 for steam at saturated at any pressure

Kb for Conventional PRV, Vapor/Gas only

Kb for Balanced PRV, Vapor/Gas only

Subcritical Flow

Under subcritical flow condition, the required effective

discharge area for a conventional and pilot type is calculated

based on the following formula:

F2 = subcritical flow coefficient which define as

r = ratio of back pressure to upstream relieving pressure

P2 = back pressure in absolute

Subcritical Flow, F2 using Figure

Sizing PSV for Liquid Service

Pressure relief valves that are designed for liquid service

require capacity certification. It can be initially sized using (SI)

Q = flow rate, liter/min

Kw = correction factor due to back pressure

Kv = correction factor due to viscosity

p1 = upstream relieving pressure, kPag

p2 = back pressure, kPag

1. Initially, Kv = 1, (non-viscous fluid)

2. The A value should be changed based on API 526

3. Calculate Reynold’s Number, R, (SI-Dimensionless)

= absolute viscosity at flowing temperature, cP

4. Then calculate the Kv based on the following figure

5. The A then recalculate based on the new Kv

6. If the A size bigger than step no.2, then the calculation should be repeated using the next larger standard orifice size

Sizing PSV for Liquid Service

Sizing PSV for Liquid Service

Pressure relief valves that are designed for liquid service

which do not require capacity certification. It can be initially

sized using (SI)

This method assumes effective coefficient of discharge,

Kd = 0.62 and 25% overpressure. Additional capacity

correction factor, Kp, is needed for relieving pressures other

than 25% overpressure.

p = set pressure, kPag

pb = total back pressure, kPag

Kp = correction factor due to overpressure

Sizing PSV for Liquid Service

Sizing PSV for Liquid Service

Sizing for Vaporizing Liquid, Fire

The following method is used to calculate the required orifice

area for PRV on vessels containing liquids that are exposed to

fire.

1. Determine the total wetted surface area

2. Determine the total heat absorption (BTU/hr)

a) Prompt fire-fighting and adequate drainage exist :

b) If it does not exist :

3. Determine the rate of vapor or gas vaporized from the

liquid ( lbs/hr) :

4. Calculate the minimum required relieving area

Wetted Area Calculation

Sphere :

Horizontal cylinder with flat ends :

Horizontal cylinder with spherical ends

Vertical cylinder with flat ends

If E < L then,

If E = L then,

Vertical cylinder with spherical ends

Vessel Sketch

A = wetted area, square feet

B = effective liquid level angle, degrees

= cos-1 [1 – (2) (E) / (D)]

Vessel Sketch

E = effective liquid level, ft, 25 ft from the flame source

Es = effective spherical liquid level, feet

up to a max horizontal dia. or 25 ft whichever

greater

Environmental Factor

The relief load (lb/hr) can be determined using :

M = relative molecular mass of the gas

A’ = the exposed surface area of the vessel, square feet

Tw = the recommended maximum wall temperature of vessel

material, R2

T1 = gas absolute temperature, at P1, oR

Vessel Contains Vapor/Gas Only

Carbon steel plate material, Tw = 593oC or 1100oF. If vessels

are fabricated from alloy material, the value of Tw should be

changed to a more appropriate recommended maximum.

The attachment files are some of the causes of overpressure that

will be discussed further. The data will be used to size the PSV

using the formula as mentioned before.

Vessel Contains Vapor/Gas Only

RD is installed at Upstream of PRV Due to

1. Safety

a. Plugging PRV inlet

b. Prevent PRV opening

2. Environment (Passing or

Leaks)

3. Cost

a. Corrosive fluid (CRM

PRV is more expensive

than CRM RD + PRV)

b. Longer overhaul

c. Extended life span

1. API RP 520, Part I – Sizing and Selection, 7th Ed, 2000

2. API RP 520, Part II – Sizing, Selection and Installation of

Pressure-Relieving Devices in Refineries 4th Ed, 1994

3. API Std 521, Pressure-Relieving and Depressuring Systems,

5th Ed, 2007

4. Crosby, PRV Engineering Handbook, 1997

5. Lesser, PRV Engineering Handbook, 2010

6. Workbook for Chemical Reactor Relief System Sizing,

1998

References