Lecture 13 Relief Sizing

ECE 4353

Chemical Process Safety

Relief Sizing

(Lecture 13)

Last Updated:1 December 2015

© LMS SEGi education group 2

LEARNING OBJECTIVES

Explain the government-industry’s responsibility for

health and safety

Evaluate the nature of hazards posed by materials

which are flammable, toxic and reactive

Identify and quantify common industrial methods to

control hazards.



Lecture 13

13.1 Pressure Relief Installations

13.2 Pressure Relief Definitions

13.3 Pressure Relief Valve Sizing for Liquid Service

13.4 Pressure Relief Valve Sizing for Vapor/Gas Service



Last Updated:1 December 2015 4

Spot the error!

13.1 Pressure Relief Installations

Valves not allowed in locations that block a relief




Spot the error!

Relief for a vessel should be on a vessel, not the inlet or outlet line

All vessels need dedicated PSV. Valves not allowed in locations that block a relief. Rupture disc and tell tale gauge are recommended before PSV to protect PSV from corrosive fluid.

Discharge pipe shall be greater in size than inlet size.




Spot the error!

Relief shall be sized for WORST case scenario Size for 3 tanks, with valves Opened.

The spring operated relief valve will open first; discharging material on top of the rupture disk. Rupture disk may not open at set point. If discharge is polymerizing, the polymer may plug discharge of rupture disk.

Relief shall be sized for WORST case scenario Size for 200 gpm




• Set pressure: The pressure at which the relief device begins to

activate.

• Maximum allowable working pressure (MAWP): The maximum

gauge pressure permissible at the top of a vessel for a

designated temperature. This is sometimes called the design

pressure.

• Operating pressure: The gauge pressure during normal service,

usually 10% below the MAWP.

• Accumulation: The pressure increase over the MAWP of a

vessel during the relief process. Expressed as a percentage of

the MAWP.

• Overpressure: The pressure increase in the vessel over the set

pressure during the relieving process. Overpressure is

equivalent to the accumulation when the set pressure is at the

MAWP.

13.2 Definitions




• Backpressure: The pressure at the outlet of the relief device

during the relief process resulting from pressure in the

discharge system.

• Simmer is the audible or visible escape of compressible fluid

between the seat and disc which may occur at an inlet static

pressure below the set pressure prior to opening.

• Blowdown is the difference between the set pressure and the

closing pressure of a pressure relief valve, expressed as a

percentage of the set pressure or in pressure units.

• Excessive built-up back pressure can cause the valve to

operate in an unstable manner. Chatter refers to the abnormally

rapid reciprocating motion of the pressure relief valve disc

where the disc contacts the pressure relief valve seat during

cycling. This type of operation may cause damage to the valve

and interconnecting piping.

13.2 Definitions










The objective of the relief valve sizing is to

determine the required relief area for the relief

device.

13.3 Pressure Relief Valve Sizing




Conventional Spring-Operated Reliefs in

Liquid Service Flow through spring-type reliefs is approximated as flow through an orifice.

Qv = u.A

13.3 Pressure Relief Valve Sizing – Liquid







• The viscosity correction Kv, corrects for the additional

frictional losses resulting from flow of high-viscosity

material through the valve.

• The required relief vent area becomes larger as the

viscosity of the liquid increases (lower Reynolds

numbers).

• This correction is given in Figure 9-2.







• The overpressure correction Kp, includes the effect

of discharge pressures greater than the set pressure.

• The overpressure correction Kp, is a function of the

overpressure specified for the design.

• Up to and including 25% overpressure, the relief device

capacity is affected by the changing discharge area as the

valve lifts, the change in the orifice discharge coefficient,

and the change in overpressure. Above 25% the valve

capacity is affected only by the change in overpressure

because the valve discharge area is constant and behaves

as a true orifice.

• This correction is given in Figure 9-3.

• Valves operating at low overpressures tend to chatter, so

overpressures less than 10% should be avoided.




As the specified overpressure becomes smaller, the correction value decreases, resulting in a larger relief area.




The backpressure correction Kb, is used only for balanced-

bellows-type spring reliefs.




Example – PSV for Liquid service

A positive displacement pump pumps water at 200 gpm at

a pressure of 200 psig.

The pump is protected by a PSV set @ 200 psig.

Assuming a backpressure of 20psig, compute the area

required

(a) 10% overpressure

(b) 25% overpressure




Co = 0.61 (conservative value)

Kv = 1.0 (Re >5000, turbulent flow)

Kp = 0.6 (at % overpressure 10%)

Kb = 1.0 as PSV is not balanced bellows




Relief vent area decreases as the overpressure increases.





Gas Service Flow through spring-type reliefs is approximated as flow through an orifice.

Define

13.4 Pressure Relief Valve Sizing – Gas service





Gas Service

P = P max + 14.7

13.4 Pressure Relief Valve Sizing – Gas service







Kb values for conventional type




Kb values for balanced bellows




Example – PSV for Vapor/Gas service

A nitrogen regulator fails and allows nitrogen to enter a

reactor through a 6-in diameter line.

The source of the nitrogen is at 70F and 150 psig.

The relief valve is set at 50 psig.

Determine the diameter of a balanced bellows spring type

vapor relief required to protect the reactor form this

incident.

Assume a relief backpressure of 20 psig.




1. Determine relief capacity required

2. Determine if flow is choked.

diatomic

max relief design pressure within the reactor







Relief Valve Orifice size










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Lecture 13 Relief Sizing