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.
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
temperature 0C
|
R717 barg
|
R22 barg
|
R134a barg
|
R410A barg
|
R407C barg
|
|
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.
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)
-
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.
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
|
Relief device capacity
in lbs/m air
|
Maximum length of discharge piping in feet based on
20barg set point
Schedule 40 pipe
|
|||||
|
1/2"
|
3/4"
|
1"
|
11/4"
|
11/2"
|
2"
|
|
|
6
|
173
|
|
|
|
|
|
|
8
|
97
|
|
|
|
|
|
|
10
|
62
|
254
|
|
|
|
|
|
12
|
43
|
176
|
|
|
|
|
|
14
|
32
|
130
|
|
|
|
|
|
16
|
24
|
99
|
|
|
|
|
|
18
|
19
|
78
|
|
|
|
|
|
20
|
16
|
63
|
212
|
|
|
|
|
25
|
10
|
41
|
136
|
|
|
|
|
30
|
7
|
28
|
94
|
|
|
|
|
35
|
5
|
21
|
69
|
|
|
|
|
40
|
4
|
16
|
53
|
209
|
|
|
|
45
|
3
|
13
|
42
|
165
|
|
|
|
50
|
2.5
|
10
|
34
|
133
|
|
|
|
60
|
1.5
|
7
|
24
|
93
|
200
|
|
|
70
|
1.5
|
5
|
17
|
68
|
147
|
|
|
80
|
1
|
4
|
13
|
52
|
113
|
|
|
90
|
1
|
3
|
10
|
41
|
89
|
|
|
100
|
|
2.5
|
8
|
33
|
72
|
252
|
|
125
|
|
1.5
|
5.5
|
21
|
46
|
161
|
|
150
|
|
1
|
4
|
15
|
32
|
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 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.
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