 |
|
| |
| Smith Flow Control Ltd. |
| |
6 Waterside Business Park
Eastways Industrial Estate
Witham Essex CM8 3YQ
United Kingdom |
| |
Tel: +44 (0)1376 517901
fax: +44 (0)1376 518720 |
sales@smithflowcontrol.com
www.smithflowcontrol.com |
|
| |
|
|
|
| Home » Applications
» Safety Relief System |
| |
 |
| |
|
| |
Whether onshore or offshore, maintenance
procedures to Safety Relief Systems on live plant cause
concern in ensuring that an open path to relief is maintained
at all times during the work exercise.
Most modern piping arrangements include spare relief capacity
enabling continuous production while such maintenance
procedures are conducted on live plant eliminating the
requirement to isolate and shut down the process.
Twin or multiple safety relief valve systems are usually
fitted with isolation block valves upstream and downstream
of each safety relief valve - it is essential to ensure
that the block valves isolating the spare relief valve
are opened BEFORE the block valves of the workpiece relief
valve are closed. This issue is a more acute problem if
the respective relief valves and isolating block valves
are distant or out of sight of each other.
Various API and ASME
codes recognise this hazard and recommend the use of interlocks
to eliminate this possibility API RP 520
(Pressure Relieving Systems for Refinery Services - Part
II Section 4- Isolation Valve Requirements) and API
RP 14E (Design and Installation of Offshore Production
Platform Piping Systems - Para 5.8b [2] - Relief Device
Piping) specifically recommend interlocks in this situation
to ensure that (over)pressure protection of the vessel
is not compromised. These recommendations are also in
accordance with the requirements of ASME - Section
VIII - Appendix M
.
Fitting a valve interlock to the upstream and downstream
isolating valves on each safety relief valve (PSV) will
ensure these recommendations are implemented. An initiating
key, issued from a secure location (Control Room) commences
the PSV changeover. Follow the Step-By-Step procedures
in the diagram. |
| |
| ....... 'apart from the possibility
of death or injury to personnel, or pollution of the locality,
what other possible impact could an over-pressure explosion
or rupture of one of your main process vessels have on
your business' ? |
| |
 |
| |
| |
Click
here to view the animation |
| |
|
| |
ILLUSTRATION STATUS:
PSV1 on line: PSV2 isolated
Block Valves V1, V2
& V4* are Locked
Open (LO.)
Block Valve V3 is Locked Closed (LC)
*Spare outlet (downstream) block valve normally
LO especially if lower pressure rated than corresponding
inlet (upstream) block valve |
| |
|
| |
Requirement:
There is always an open path to relief.
(Inlet block valves V2
& V3
must never be closed at the same time) |
| |
|
| |
Maintenance Task:
To commission PSV2 into service.
To isolate and permit removal of PSV1 for testing/servicing.
|
| |
|
| |
Work Sequence:
1. |
Key 'D' is issued
from the Control Room to initiate the Permit |
2. |
Key 'D' unlocks and enables
the opening of spare inlet block valve V3 |
3. |
V3
is operated to open and is LO by removing
Key 'C' PSV2 is now on line. |
4. |
Key 'C'
is transferred to unlock and close inlet block
valve V2 |
5. |
V2
is LC by removing Key 'B' |
6. |
Key 'B'
is transferred to unlock and close outlet
block valve V1 |
7. |
V1
is LC by removing Key 'A' |
|
| |
The return of Key 'A'
to the Control Room closes-out the work permit and
enables the issue of instructions for removal of
PSV1 for testing and servicing. |
| |
|
| |
Twin PSV Changeover Sequence
It should be noted this interlock arrangement further
ensures that the higher pressure rated inlet block
valve V2
is closed before its corresponding
lower pressure rated outlet block valve
(V1).
Conversely, this key interlock arrangement would
also ensure opening of low pressure outlet block
valves before opening of high pressure
inlet block valves. |
|
| |
| |
 |
|
|
|
You can download an
animation showing a typical Safety Relief
Valve System by clicking on the icon. |
|
|
|
| |
|
| |
|
