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ASAC REFRIGERATION DIVISION
GUIDELINES FOR PRESSURE RATINGS OF VESSELS & FITTINGS
Colin Hewetson 4/5/05
1) Design & Maximum Allowable Working Pressures
On a few occasions we have had an issue where some of the system components have been ordered with a
MAWP less than the system required MAWP. There is always some confusion in terminology used in
pressure rating of components e.g. valves, pipe, flanges, vessels. We see many different terminologies in
common usage e.g.
- Design pressure
- Maximum working pressure
- Maximum allowable working pressure
- Working pressure
- Rated pressure
- Service pressure
Essentially they refer to one and the same definition; they all refer to the MAWP of a component or a system.
MAWP is the maximum pressure at which a system or its component is designed to operate safely. Another
term sometimes used is Maximum operating pressure MOP, this is the maximum pressure a component or
system is normally operated and is usually 10-20% below the MAWP. Design pressure & MAWP are
normally equal.
The design pressure of a vessel or component will not be less than the MAWP & the materials pressure rating
shall correspond to the equivalent temperature rating. For example in accordance to ASME B 16.5-1996, a
class 150 flange for a temperature range of -29 to + 38degC has a pressure rating of 19.6barg, however at
93degC only 17.9barg. Whenever design engineers order pressure vessels in particular, they must specify the
MAWP & minimum/maximum operating temperatures to the supplier in order to ensure the correct material is
selected & priced.
In accordance to the relief valve settings as detailed in Para 6, allowance has to be made in calculating the
design MAWP for the margins between the normal high side or low side operating pressures and the
switching pressure of any pressure safety devices such as cutouts or safety relief valves. In this regard the
suggested minimum design MAWP as detailed in 3.1, may have to be increased by 10-20% to make due
allowance for the switching margins so as to arrive at a MAOP below the MAWP at which the protective
devices are set. Safety relief valves generally start to open at 10% below the set point & the relief set point
will be the MAWP.
2) MAWP
The refrigeration system & any components therein should be designed so the MAWP is never exceeded
either when the system is working or shutdown. In this regard the design engineer must take account of
- The maximum expected operating pressures for the refrigerant for the application design.
- The corresponding temperature rating for the refrigerant for the application design
- The correct ASME or ASTM material grade for the application design pressure & corresponding
temperature.
- Pressure increase due to NCG (non condensable gases)
- The ambient temperature or plant room temperature wherever equipment is located.
- The margins between the normal high side or low side operating pressures and the switching pressure
of any pressure safety devices such as cutouts or safety relief valves.
- The relief valve set point
- The method of defrosting
- Solar radiation particularly on condensers & liquid receivers installed on roofs where in Saudi Arabia
metal surface temperatures in excess of 70degC can occur.
- Fouling of heat exchange equipment which will result in elevated shaft power + temperatures in order
to transfer the originally designed heat load.
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3) System Design pressure MAWP
The pressures to be used in calculating MAWP for the system & its components shall be in accordance to the
corresponding refrigerant saturated vapour temperature for the refrigerant used at the maximum design
ambient temperature of the area in which the plant is to be installed. This applies to both the high & low side
of the system. In Saudi Arabia the design summer temperature is between 40-50degC. Whilst ideally the
whole system should be designed for the highest pressure, commercially this may not make economic sense
& the low pressure side may be designed for a lower pressure so long as it incorporates separate overpressure
protection against rupture e.g. safety relief valve set at a set pressure equal to the low side design pressure
(MAWP).
3.1) Suggested minimum MAWP for system & components operating in Saudi Arabia
Type of system
Min design
R717 R22
R134a R410A R407C
temperature barg barg
barg
barg
barg
0C
Air cooled high side
65
30
26
18
41
27
Water cooled high side
50
20
20
13
30
19
Low side system*
35
13
13
8
20
13
* Based on installing a safety relief device in the low side set at design low side MAWP.
Plant designers should take into account that during prolonged plant shut down the low side of the
refrigeration system will equalize at the high side pressure corresponding to the ambient temperature, unless
the system is pumped down & the refrigerant stored in the HP receiver.
Plant designers should carefully check all components to ensure the manufacturers MAWP will not be
exceeded under the worst conditions as per 2 & 3, & certainly design MAWP should not be less than any
installed protective devices set point in the system to which the component is applied. For example high stage
compressor oil separators may be designed for MAWP of 23barg, so for an air cooled R717 system with a
design MAWP of 30barg a special quotation for higher MAWP pressure vessels & Nodular iron or Cast Steel
compressor housings will have to be obtained from the supplier, or the application will necessarily have to be
changed to a water cooled system. Where the high side safety relief valve relieves to the low side or suction
side of the compressor, the low side safety relief valve must be sized correctly for the discharge flow of both
the high & low side.
4) Pressure vessels
All pressure vessels must be designed to comply either with ASME V111 Div 1 or PED. The MAWP will be
calculated in accordance to criteria established under 2 & 3 above. Pressure vessels cover HP & LP liquid
receivers, oil separators, oil coolers, economizers, intercoolers, surge drums, suction line liquid KO pots,
condensers, compressors, air coolers, pipe manifolds, water cooled condensers, air cooled condensers.
Generally pressure systems are only designed to comply with a particular code, this does not mean they are
actually manufactured to that code. If the pressure system is required to be manufactured to the code
additional costs will apply; the supplier must be advised if the system has to be manufactured to the particular
code & additional price requested for this plus the relevant certification. Manufacturers brochures need to be
carefully studied as the design pressure or MAWP used will correlate to the maximum design temperatures
relevant to Europe of USA e.g. an air cooled system in UK might be based on an ambient of 30degC whilst in
Saudi Arabia the design ambient may be 45-50degC. This is particularly so for booster compressors & pump
surge drums where the MAWP is designed for 10barg. In such instances the manufacturer must be contacted
to provide the price add for a higher MAWP.
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5) Flanges & components
Flanges must be selected by class rating. For all low side fittings class 150 minimum must be used so long as
the pressure rating is within the temperature range for that class. For all high side fittings class 300 minimum
must be used so long as the pressure rating is within the temperature range for that class. For R744, R404a,
R410a higher ratings must be used due to the higher pressures corresponding to the high side saturated
temperature, for these refrigerants class 400 fittings should be used. Flanges must be weld neck with captive
gaskets using tongue & groove design. The gasket materials must be compatible with refrigerant & type of oil
intended to be used. Bolt & nut material must be in accordance to the pressure rating & of adequate length to
ensure no more than 2-3 threads exceed the nut face. Welding should be class 1 arc or MIG welding down to
35mm carried out by certified welders. Butt welded joints below 37mm should be welded using Oxy –
Acetylene.
6) Correlation between MAWP & set points
The system set points should correspond to the following pressures
- System MAWP x 1.3 = System strength test pressure
- System MAWP x 1.1 = Leak test pressure (Relief valves must be removed & connections blinded)
- System MAWP x 1.1 = Relief valve fully open
- System MAWP
= Relief valve set point at which relief valve begins to open
- System MAWP x 0.9 = Pressure safety switch operates
- System MAWP x 0.8 = Maximum operating pressure (MOP or MAOP) under normal conditions
So for an R717 water cooled system that at design conditions in Saudi Arabia would normally operate at
40degC condensing temperature, the minimum design MOP would be 15barg, the minimum design MAWP
would be 20barg. This then allows for the various switching differentials of all the safety devices as well as
protection during plant shutdown in peak summer design ambient temperatures.
7) Relief valves
Relief valves are designed to prevent pressure in a vessel or pressure system from rising above a safe limit
when operating safety devices such as HP cutout fail, or when the vessel or system is exposed to excessive
heat.
If a vessel is filled with liquid with no vapour space above it, a small rise in temperature will cause a rapid &
excessive rise in pressure due to the expansion of the incompressible refrigerant liquid with likely rupture of
the vessel. If as it should, the vessel say a HP receiver or Surge drum, contains both liquid & vapour the
pressure will rise in accordance to the temperature pressure saturation characteristics of the applicable
refrigerant. If the density of the liquid vapour mixture in the vessel exceeds the critical density of the
applicable refrigerant, an increase in temperature will cause an increase in the % of liquid in the vessel until
the vessel is completely filled with liquid. A small increase in temperature beyond this point will result in an
excessive & rapid increase in pressure. Such condition can occur at temperatures well below the refrigerant
critical temperature as a result of exposure of the vessel to excessive heat emanating from fire or solar
radiation. If pressure builds up high enough to cause the vessel or tube to rupture, large quantities of liquid
refrigerant will be released resulting in rapid depressurization of the system, freezing up of heat exchangers.
The sudden reduction of pressure causes the liquid to vapourize almost instantly with explosive results.
So the purpose of the relief valve is to release the pressure at a controlled rate maintaining a safe pressure in
the vessel or system. The most popular type of relief valve used for refrigeration is the direct spring loaded
"pop" type which contains a piston & spring. At the relief valve set point the force exerted by the spring is
equal to the force exerted by the refrigerant pressure. As the refrigerant pressure increases due to an abnormal
condition, the pressure increases slowly above the relief valve setting & the piston begins to lift until there is
enough flow to pop the piston open & provide full discharge. The pressure above the valve set point at which
the piston is fully open depends on the valve design. Since the flow rate is measured at a pressure of 10%
above the set point it is necessary that the valve opens within this 10%.
The valve operates on a fixed differential from inlet to outlet & is therefore affected by back pressure.
There are a number of different types of relief devices
- Fusible plugs (limited to vessels < 3ft3)
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-
Rupture disk (generally used for low pressure systems & for installation downstream of a relief valve
to monitor relief valve integrity).
Pressure relief valve
There are various code compliances
- ISO standard
- API RP520
- API RP521
- API 2000
- ASME section V111
- BS4434
- ASHRAE 15-78/ANSI B9.1
Generally the API codes are only used for Chemical, Petrochemical & refinery requirments where a fire
hazard exists, in such cases the calculated relief rate is based on the actual refrigerant being relieved and not
equivalent air flow rate. In some instances the more stringent API2000 venting atmospheric & low pressure
storage tanks may be specified for the calculation of fire relief rates, even when the installation is to be in
accordance to API RP520. For these codes the discharge rate would be calculated on the greater pressure due
to
- Fire condition
- Burst tube condition
- Blocked discharge condition
- Condenser water or air cooled fan failure
ASHRAE & BS codes use identical methods of calculation of the relief rate based on an equivalent air rate in
lbs/m or Kg/s. For general purpose food refrigeration systems these codes would normally be applied.
In calculating the discharge capacity of the pressure relief valve based on equivalent air flow the following
formulae applies.
C = f.D.L where
C = Minimum required discharge rate of relief valve in lbs/min
D = Outside diameter of vessel in feet
L = Length of vessel in feet
f = Constant dependant upon the refrigerant as follows
R717
= 0.5
R134A/22/500
= 1.6
R13/13B1/14
= 2.5
All other refrigerants = 1.0
For example a vessel of 5' diameter x 10'0" long for R22 (f = 1.6) the discharge rate would be
5 x 10 x 1.6 = 80lbs/m air.
For R717 (f = 0.5) the discharge rate would be 5 x 10 x 0.5 = 25lbs/m air. This lower rate for R717 is due to
its high latent heat of vapourization with greater cooling effect.
The relief valve set point will be equal to the MAWP; to prevent nuisance discharge & loss of refrigerant
during normal operating conditions the set point should be above the normal system operating pressure.
A relief valve for a typical HP R717 receiver for example operating at a design of 40 degC, would be based
on a set point of 20barg (table 3.1) The corresponding saturation pressure for 40degC is 15barg. In accordance
to table 3.1 the minimum recommended MAWP is 20barg which is 33.3% above the normal operation MAOP
saturation pressure. The differential between the normal operating pressure MAOP & MAWP should be at
least 20%. So it can be seen that table 3.1 recommended MAWP is satisfactory for an MAOP of 15barg in
relation to the protective devices set point.
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For vessels greater than 10ft3 most codes recommend a three way valve with two relief valves. This is just
plain common sense in any event & system designers should always provide three way valves + two relief
valves. If only one relief valve is provided there is very high potential to lose the entire refrigerant charge in
the event of relief valve leakage or lifting due a system overpressure situation arising. This scenario would
disable the plant until such time as the valve is inspected, repaired as necessary & recalibrated & the system
recharged.
With two relief valves + valve manifold, the system can still operate by closing off the faulty or lifted valve
& opening up the port to the standby valve. The faulty or lifted valve can then be inspected, repaired &
recalibrated without interrupting plant operation.
Once a relief valve has popped, it should not be allowed to remain in operation until it is checked &
calibrated.
Under no circumstances must any shut off valve be placed between the system or vessel protected by a relief
valve & the relief valve itself. The danger of doing so is that the valve may accidentally be left in the closed
position, which removes the protection provided by the relief valve & leaves the system totally unprotected
against overpressure.
When ordering relief valves the supplier must be provided with the set pressure point required, the discharge
relieving capacity in lbs/m as calculated, inlet & outlet port sizes, connection details, refrigerant & oil type
used. A calibration certificate is essential & this must be specified when ordering relief valves.
Standard relief valve settings provided by the manufacturer may not be suitable for the system design MAWP.
The relief valve set point must be ordered to suit the system design MAWP, do not order relief valves with a
standard setting which is "near enough" to the system design MAWP. Generally manufacturer's standard set
points are 150/175/200/225/250/275/300/325/350/400/425/450.
8) Relief valve discharge piping
Due to the toxic or flammable properties of many refrigerants & for safety of plant operators, relief valves
should discharge outside of the equipment space to atmosphere or in the case of Petrochemical complexes to
the flare stack. When the relief valve is discharging refrigerant, back pressure will build up in the discharge
piping which may prevent the device from performing its rated discharge capacity. The effective discharge
rate of a relief valve depends on both the valve orifice diameter plus the lift of the valve piston which in turn
depend upon the pressure differential across the valve. As the back pressure increases so the flow rate will
reduce. The amount of flow reduction will depend upon the valve design & amount of back pressure.
ANSI B9.1 code permits a maximum back pressure through the discharge piping of 25% of the inlet pressure
while the valve is discharging at its rated capacity. Knowing the set pressure & capacity of the relief valve, the
length of discharge piping for each pipe size can be calculated using the following formulae.
L= 9P2d5
C2
Where
L = Length of discharge pipe in feet
P = 0.25 [(set pressure x 1.1) + 14.7]
d = Pipe nominal bore in inches
C= Minimum required discharge capacity in lbs/m air calculated from 7.0
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In order to minimize the time in calculating I enclose pipe data in the following table based on 20barg valve
set point.
Relief device
capacity in
lbs/m air
Maximum length of discharge piping in feet based on 20barg
set point
Schedule 40 pipe
1/2"
6
8
10
12
14
16
18
20
25
30
35
40
45
50
60
70
80
90
100
125
150
173
97
62
43
32
24
19
16
10
7
5
4
3
2.5
1.5
1.5
1
1
3/4"
1"
11/4"
11/2"
2"
254
176
130
99
78
63
41
28
21
16
13
10
7
5
4
3
2.5
1.5
1
212
136
94
69
53
42
34
24
17
13
10
8
5.5
4
209
165
133
93
68
52
41
33
21
15
200
147
113
89
72
46
32
252
161
112
9) API safety valve selection
To determine the relief valve discharge rate, several factors must be taken into account as mentioned in Para
7, specifically
a) Blocked outlet
b) Condenser water failure
c) External fire
The required relief rate will be determined from the largest flow likely to arise from any of the above
situations arising or the sum of any rates which may contribute simultaneously.
With Centrifugal machinery it is not normally possible for the relief pressure to be attained as a result of a) or
b), as the compressor will surge before the set pressure is reached. However with positive displacement
compressors the pumping rate of the compressor at the relief valve set point must be checked. In most cases
the largest relief flow rate will arise as a result of an external fire & the relief rate can be calculated in
accordance to the formulae contained in API RP520 as follows.
a) Heat input from fire
Q = 21000 x f x A0.82 where
Q = Heat input in Btu/h
f = Environment factor. f = 1 for bare vessels or < 1 if the vessel is fitted with fire resistant
insulation.
A = Wetted surface of vessel in ft2
b) Relief rate
The relief rate is determined by dividing the heat input by the latent heat of vapourization of the
refrigerant at the accumulated pressure which is set pressure plus over pressure. Overpressure means
the pressure rise over & above the MAWP. Most codes allow the pressure in a vessel to rise by 10% or
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MAWP x 1.1 at relief conditions other than caused by fire. For fire exposure ASMEV111 Div 1 &
similar codes allow the pressure to rise by 20% or MAWP x 1.2. ASME V111 Div2 only allows a rise
of 10% under fire conditions even though the vessels are designed for higher stresses under the code.
So the latent heat will be calculated on MAWP + 10 or 20% as applicable under the code.
The formulae is
W = Q/L where
W = Relief rate in lbs/h
Q = Heat input in Btu/h
L = Latent heat Btu/lb
10) Relief valve sizing
Relief valve sizing where the discharge rate has been calculated in lbs/min air using ASHRAE formulae is just
a matter of reference to a manufacturers catalogue, Henry, Herl, Danfoss, Parker all provide a good range of
safety relief valves with selection information.
For API relief valves, compliance to the code is mandatory so the above manufacturers generally do not
comply. Manufactures such as Bailey Birkett, Crosby, GEC Elliot Farris all design their valves to comply
with API RP520 in terms of dimensional aspects of inlet – outlet connections, orifice area, material &
certification compliance. These manufacturers provide formulas for calculation of the orifice area in
accordance to API.
The basic formulae used where the relief rate has to be calculated in lbs/h of refrigerant vapour is
A = W x square root of TZ
C x KD x PI x KB x square root of M
Where
A = Effective discharge area of the valve in in2
W = Flow through valve in lb/h refrigerant
KD = Coefficient of discharge, a value obtained from the valve manufacturer, but typically 0.9 – 0.975
C = Coefficient determined by the ratio of specific heats Cp/Cv of the vapour. These values are also
provided by the valve manufacturer but if not known use a conservative value of for C of 315.
KB = Capacity correction factor for imposed back pressure, value from valve catalogue. KB value
where the valve discharges straight to atmosphere is 1.
M= Molecular weight of the refrigerant or vapour can be found in most refrigerant tables.
T = Absolute temperature of the refrigerant vapour at the accumulated pressure e.g. decC + 280 or
degf + 460.
Z = Compressibility factor for deviation from an ideal gas, to be evaluated at valve inlet conditions. If
Z is not known use a safe value of Z = 1.
PI = Upstream pressure valve inlet in absolute lbfa or bara, e.g. valve set pressure lbf/in2 +
overpressure % either 10 or 20% +14.7
The relief valve piping should not be smaller than the relief inlet connection, therefore under any code used
the relief rate in lbs/h must be calculated & the pipe size selected prior to estimating the cost or designing the
vessels. The allowable pressure drop in the relief valve inlet piping should not exceed 3% of the set pressure.
For the relief valve discharge piping, the pipe size must not be smaller than the valve outlet connection & the
pressure drop should not exceed 10% of the valve set pressure. Where several relief valves are piped upto a
common header it is important to design the header for the relief valve with the largest set pressure & the
pressure drop is low enough so as not to influence the relief rates or opening of the other relief valves on the
same header.