safety valves CIBSE
Transcript of safety valves CIBSE
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Andy LucasTechnical Development Manager CRANE BS&U
CRANE BS&U supporting the
Building ServicesIndustry
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Crane Co founded in 1855 by Richard Teller Cranewho made the following resolution -
“I am resolved to conduct my business inthe strictest honesty and fairness; to avoid
all deception and trickery; to deal fairly with
both customers and competitors; to be
liberal and just towards employees; and to put my whole mind upon the business”
Crane Limited founded in Ipswich in 1919
Crane Building Services & Utilities created 2009
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Building Services
Water UtilitiesGas Utilities
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CRANE BS&U 2010 sales £110m
Gas Utilities Building Services
Water Utilities
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industry organisations
•CIBSE
•SoHPE
•BSRIA
•CSA
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6 Andy LucasTechnical Development Manager CRANE BS&U
CIBSE appro ved CPD
Introduction to Safety Valves
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An introduction to Safety Valves
To give an understanding of Safety Valves
used in Building Services including •sizing
•installation
•maintenance
Title
Objective
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What are they?
Safety Valves
Definition
(PED)
PED
Standard
Pressure Equipment Directive 97/23/EC•the regulation covering safety valves in Europe•safety valves are classified PED category IV
EN ISO 4126 - joint European & ISO standard
BS6759 – valve manufactured to BS6759 are deemed tocomply with EN ISO 4126 if no design changes
‘a valve which automatically, without the assistance of anyenergy other than that of the fluid concerned, discharges aquantity of the fluid so as to prevent a predetermined safepressure being exceeded, and which is designed to reclose
and prevent further flow of fluid after normal pressureconditions of service have been restored’
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Safety Valves
Safety valves, as the name implies, have a
specific function
to protect equipment & personnel
EN ISO 4126 only refers to safety valves & covers
•safety valves
•relief valves
•pressure relief valves
•safety relief valves
What are they?
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Safety Valves
Why we need one?
•blocked discharge
•external heat
•thermal expansion
•failure of pipeline component, ie control valve
4 main reasons
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Safety Valves
Why we need one?
•blocked discharge
4 main reasons
safety valve
no flow
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Safety Valves
Why we need one?
•blocked discharge
•external heat
4 main reasons
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Safety Valves
Why we need one?
•blocked discharge
•external heat
•thermal expansion
4 main reasons
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Safety Valves
Why we need one?
•blocked discharge•external heat
•thermal expansion
•failure of pipeline component, ie control valve
4 main reasons
no flow
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Safety Valves
Terminology
Set pressure the pressure at which the valves starts to open
measured at valve inletnormally 1.1 x working pressure or workingpressure + 0.7 bar for water whichever is thegreater
Overpressure pressure at which valve has to achieve its fulldischarge capacity normally set pressure +10%
Accumulation pressure increase over the maximum working
pressure of the system during discharge through
the safety valve
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Safety Valves
Operating states
1. Equilibrium
2. Fully open
3. Fully closed
there are 3 operating states for safety
valves
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Safety Valves
Terminology
Blowdown the pressure difference between set
pressure and pressure at which the valvereseats expressed as a % of the setpressure
Reseat Pressure pressure at which valve is fully closed
Working Pressure pressure at which the system being
protected normally operates
Discharge Capacity the amount of water / gas/ vapour the
valve will pass at a given pressure
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Safety Valves
Equilibrium
forces acting to close are in
equilibrium (balance) with the forcesacting to open and the seat & disc are just in contact
defined in EN ISO 4126 as the Set
Pressure
this is the point when flow is about tostart
in practice flow commences as theequilibrium point is reached
P system
P spring
P system = P spring
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Safety Valves
Fully open
the position the valve must achieve to
pass its rated capacity at its specifiedoverpressure
note - this is not the Set Pressure
different designs of valve have differentrelationship between flow & pressure
some designs have a rapid increase inflow for a small increase in pressure -others offer a gradual increase
P system +
overpressure
P spring
P system + overpressure = P spring
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Safety Valves
Fully closed
the position when the valve has re-seated
ie fully closed - nil leakagethe difference between the re-seatpressure & set pressure is often referred toas the Blowdown Pressure
the blowdown pressure will depend onvalve design, the faster the flow increaseson opening the lower the blowdownpressure is P system
P spring
P re-seat = P system - blowdown
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Safety Valves
Direct Spring Safety Valve
within Building Services the
Direct Spring Safety valve isgenerally used
the spring is pre-compressed toapply force downwards on the
disc holding it onto the seat tomaintain a pressure seal
if the system pressure increasesthe spring force is exceeded &flow commences
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Safety Valves
Direct Spring Safety Valve
seat
disc
stem
adjuster
easing lever
spring
body
bonnetDirect Spring
with easing lever
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Safety Valves
Direct Spring Safety Valve
seat
disc
stem
adjuster
topworks enclosure
spring
body
bonnet
Direct Spring
with topworks enclosure
lock nut
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Safety Valves
Combined Pressure & Temperature
Direct Spring Safety Valve with
temperature funtion
the spring is pre-compressed toapply force downwards on thedisc holding it onto the seat to
maintain a pressure seal
if the system pressure ortemperature increases the springforce is exceeded & flowcommences temperature
sensor
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Safety Valves
seat
disc
stem
adjuster
easing lever
spring
body
bonnetDirect Spring CombinedPressure & Temperature
with easing lever
Combined Pressure & Temperature
temperature sensor
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Safety Valves
Sizing
sizing affects how a Safety Valve performs
correct sizing of Safety Valves is important
•undersize valves may not relieve sufficientquantity of system media to prevent pressurebuild-up
•oversized valves will partially lift at set pressure& then re-seat so could cause ‘chattering’ of thedisc damaging the seat / disc surfaces
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Safety Valves
Sizing
requirements for sizing Safety Valves are
What is the application
What is the set pressure
What is the discharge capacity required
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Safety Valves
Sizing
requirements for sizing Safety Valves are
What is the application
•system media, ie water / steam etc
•operating temperature
•operating pressure
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Safety Valves
Sizing
requirements for sizing Safety Valves are
What is the application
What is the set pressure
•the pressure at which the valves starts to open
measured at valve inlet
normally 1.1 x working pressure
or
working pressure + 0.7 bar for water whichever
is the greater
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Safety Valves
Sizing
requirements for sizing Safety Valves are
What is the application
What is the set pressure
What is the discharge capacity required
•the amount of water / gas / vapourrequired to discharge at a givenpressure
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Safety Valves
Installation
installed position of the Safety Valve affects how theSafety Valve performs
mount vertically
avoiding
•long pipework runs to Safety Valves
•valves between system & Safety Valve
•fittings / bend between Safety Valve and system
•installation near to pipework lateral connections
•inverted outlet
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Safety Valves
Installation
•long pipework runs to Safety Valves
pipework losses betweenprotected system & SafetyValve should not be greater
than 3% of maximumdischarged capacity pressure
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Safety Valves
Installation
•long pipework runs
•valves between system & Safety Valve
under no circumstancesshould valves be installed
between safety Valve &protected system
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Safety Valves
Installation
•long pipework runs
•valves between system & Safety Valve•fittings / bend between Safety Valve and system
fitting etc between
Safety Valve & systemaffect discharge
capacity & should beavoided
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Safety Valves
Installation
•long pipework runs
•valves between system & Safety Valve•fittings / bend between Safety Valve and system
•installation near to pipework lateral connections
flow into lateralpipework affects system
pressure at SafetyValve inlet
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Safety Valves
Installation
•long pipework runs
•valves between system & Safety Valve•fittings / bend between Safety Valve and system
•installation near to pipework lateral connections
•inverted outletsystem media trappedin outlet creates a backpressure on the SafetyValve disc affecting set
pressure
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Safety Valves
Maintenance
regular maintenance ensures
•Safety Valve will work when required
•life of valve
•system is correctly protected from over pressure
•compliance with insurance company requirements
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Safety Valves
Maintenance
•the mechanical operation should be checked at three monthlyintervals by manually operating the easing lever
see installation instructions supplied with valve
•to avoid unnecessary strain on the easing gear, the valve should
be under a pressure of not less than 75% of its set pressure•where arduous conditions of service exist, more frequent testing
may be required
It is the responsibility of the user to establish thefrequency of manual testing
maintenance regime should include
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Safety Valves
Maintenance
before removing the valve, steps should be taken toensure that the system has been de-pressured
the set pressure of Safety Valves should be
checked every twelve months
•this can be carried out in situ
•usually difficult on site as gauges need to be
calibrated and system pressure needs to bevaried to check set pressure
•by removal of the Safety Valve to a test facility
Introduction to Safety Valves
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40 Andy LucasTechnical Development Manager CRANE BS&U
CIBSE appro ved
Introduction to Safety Valves