Download - 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.
Last Updated:1 December 2015
© LMS SEGi education group 3
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
© LMS SEGi education group 4
Last Updated:1 December 2015 4
Spot the error!
13.1 Pressure Relief Installations
Valves not allowed in locations that block a relief
Last Updated:1 December 2015
© LMS SEGi education group 5
Last Updated:1 December 2015 5
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.
Last Updated:1 December 2015
© LMS SEGi education group 6
Last Updated:1 December 2015 6
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
Last Updated:1 December 2015
© LMS SEGi education group 7
Last Updated:1 December 2015 7
• 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
Last Updated:1 December 2015
© LMS SEGi education group 8
Last Updated:1 December 2015 8
• 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
Last Updated:1 December 2015
© LMS SEGi education group 9
Last Updated:1 December 2015 9
Last Updated:1 December 2015
© LMS SEGi education group 10
Last Updated:1 December 2015 10
Last Updated:1 December 2015
© LMS SEGi education group 11
Last Updated:1 December 2015 11
The objective of the relief valve sizing is to
determine the required relief area for the relief
device.
13.3 Pressure Relief Valve Sizing
Last Updated:1 December 2015
© LMS SEGi education group 12
Last Updated:1 December 2015 12
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
Last Updated:1 December 2015
© LMS SEGi education group 13
Last Updated:1 December 2015 13
Last Updated:1 December 2015
© LMS SEGi education group 14
Last Updated:1 December 2015 14
• 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.
Last Updated:1 December 2015
© LMS SEGi education group 15
Last Updated:1 December 2015 15
Last Updated:1 December 2015
© LMS SEGi education group 16
Last Updated:1 December 2015 16
• 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.
Last Updated:1 December 2015
© LMS SEGi education group 17
Last Updated:1 December 2015 17
As the specified overpressure becomes smaller, the correction value decreases, resulting in a larger relief area.
Last Updated:1 December 2015
© LMS SEGi education group 18
Last Updated:1 December 2015 18
The backpressure correction Kb, is used only for balanced-
bellows-type spring reliefs.
Last Updated:1 December 2015
© LMS SEGi education group 19
Last Updated:1 December 2015 19
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
Last Updated:1 December 2015
© LMS SEGi education group 20
Last Updated:1 December 2015 20
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
Last Updated:1 December 2015
© LMS SEGi education group 21
Last Updated:1 December 2015 21
Relief vent area decreases as the overpressure increases.
Last Updated:1 December 2015
© LMS SEGi education group 22
Last Updated:1 December 2015 22
Conventional Spring-Operated Reliefs in
Gas Service Flow through spring-type reliefs is approximated as flow through an orifice.
Define
13.4 Pressure Relief Valve Sizing – Gas service
Last Updated:1 December 2015
© LMS SEGi education group 23
Last Updated:1 December 2015 23
Conventional Spring-Operated Reliefs in
Gas Service
P = P max + 14.7
13.4 Pressure Relief Valve Sizing – Gas service
Last Updated:1 December 2015
© LMS SEGi education group 24
Last Updated:1 December 2015 24
Last Updated:1 December 2015
© LMS SEGi education group 25
Last Updated:1 December 2015 25
Kb values for conventional type
Last Updated:1 December 2015
© LMS SEGi education group 26
Last Updated:1 December 2015 26
Kb values for balanced bellows
Last Updated:1 December 2015
© LMS SEGi education group 27
Last Updated:1 December 2015 27
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.
Last Updated:1 December 2015
© LMS SEGi education group 28
Last Updated:1 December 2015 28
1. Determine relief capacity required
2. Determine if flow is choked.
diatomic
max relief design pressure within the reactor
Last Updated:1 December 2015
© LMS SEGi education group 29
Last Updated:1 December 2015 29
Last Updated:1 December 2015
© LMS SEGi education group 30
Last Updated:1 December 2015 30
Relief Valve Orifice size
Last Updated:1 December 2015
© LMS SEGi education group 31
Last Updated:1 December 2015 31
Last Updated:1 December 2015
© LMS SEGi education group 32
Last Updated:1 December 2015 32
Last Updated:1 December 2015
© LMS SEGi education group 33
Last Updated:1 December 2015 33