This document discusses the anatomy of a process isolation by examining a scenario where a skillet blind was installed downstream of a double block and bleed valve isolation during a pipe tie-in project. While the isolation initially appeared safe from the piping and instrumentation diagram view, examining the mechanical view revealed that high pressure gas could be trapped in three sections, including the flange to be opened for blind removal. The revised isolation plan accounted for venting and draining trapped volumes to safely remove the blind and reduce hazards like unexpected gas releases or exposure to toxic gases. The key lessons are that closed valves can contain significant trapped volumes, especially at high pressure, and a full understanding of both process and equipment is needed for safe isolations.

1. Anatomy of a process isolation
Michael Ford

2. Anatomy of a process isolation
Scenario:
• A new project pipe section is ready to be commissioned
into existing plant process piping.
• A skillet blind was installed to obtain positive isolation
during a project tie-in at a valve. The skillet was
installed downstream of a proven double block and
bleed valve isolation. This safely isolated the new
section of pipe from existing live process.
• The downstream work has been completed, the PSSR
signed off, and now you have the task to plan the safe
removal of the skillet blind, using the proven double
block and bleed upstream isolation.
Your task is to review the isolation and determine if it is
safe for the mechanics to loosen the flange bolts to remove
the blind.
Michael Ford

3. IS THIS A SAFE
ISOLATION?
Michael Ford
Typical isolation plan:
1. Close upstream valve 1.
2. Close downstream valve 2.
3. Open Center bleed valve 3 to vent trapped
pressure and drain liquids. Close vent valve 3.
4. Ensure downstream valve 4 is open.
4. Issue permit and handover for blind
removal.
5. Periodic checks that no pressure buildup at
vent valve 3.
6. Begin removal of flange bolts

4. The answer from the Piping and
Instrument Diagram view is yes, the
isolation conforms to typical process
isolation standards and practices.
Now, let’s look at it from a mechanical
view instead of the P&ID…
Michael Ford

5. Michael Ford

6. Michael Ford

7. Does this view look different than the view from the P&ID?
Michael Ford

8. Pressure is trapped in 3 sections within this isolation,
including the flange to be opened to remove the blind
Michael Ford

9. How much gas can actually be
trapped inside of these areas?
Can we just ‘crack’ open the flange
slowly to vent the trapped gas?
In this 24” system, the volume in the
2nd isolation valve and outlet adaptor
is less than 10 cubic feet.
Michael Ford

10. But 10 cubic feet of volume assumes
atmospheric pressure, the system
pressure before the isolation was
1,300 psi - That is equal to 90
atmospheres.
At this pressure, 10 Actual Cubic Feet
becomes almost 870 Standard Cubic
Feet of wet well gas.
Michael Ford

11. How much gas was trapped inside of 2nd isolation
valve?
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12. Now let’s combine the P&ID and
mechanical two views and use the
information to reduce hazards for this
isolation and process containment
flange break.
Michael Ford

13. Michael Ford
Revised isolation plan:
1. Close upstream valve 1.
2. Close downstream valve 2.
3. Open Center bleed valve 3 to vent trapped pressure and drain liquids.
4. Close vent valve 3.
5. Re-open downstream isolation valve 2.
6. Open bleed valve 3 until venting stops, then close.
7. Close downstream isolation valve 2.
8. Open cavity drain valve 2b to drain any liquids trapped in bottom.
9. Verify valve 1 integrity by monitoring bleed valve 3 for PBU.
10. Consider venting cavity 1b on upstream isolation valve 1.
11. Periodic checks that no pressure buildup at vent valve 3.
12. Issue PTW and begin removal of flange bolts.
If toxic gas (H2S) N2 purge valve 2 using vent/drain body ports with valve
2 in partially open position, then re-close valve 2.

14. Anatomy of an isolation
What were the hazards?
1. Trapped high pressure
2. Potential unexpected release of flammable gas to
atmosphere in the presence of pipefitters.
3. Potential exposure to:
1. Direct blast of gas to personnel, ejected solids at high
speed.
2. H2S and Benzene inhalation
3. NGLs trapped in valve start flashing off with the trapped
gas release.
4. Large volume of venting, range of LEL in area
5. Ignition, tools used are spark resistant, not spark proof
Michael Ford

15. Anatomy of an isolation
What did we learn?
1. Closed Ball valves can contain a significant volume of
trapped gas at high pressure.
2. Larger valve sizes and higher pressure increase the
volume of the trapped gas.
3. Many valves have body vents and drains.
4. Isolation policies or procedures may not recognize
these facts, and they may not be discovered during
a risk assessment.
Michael Ford

16. Anatomy of an isolation
What did we learn?
5. We do not have to expose people to trapped
pressure when breaking a flange, or being unaware
of trapped pressure within a valve.
6. Planning an isolation requires understanding of both
the process and the equipment.
7. Hidden hazards can be discovered in a risk
assessment only if all the knowledge is at the table
(Ops, Eng, Maint, HSE)
Michael Ford

17. Anatomy of an isolation
QUESTIONS?
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Michael Ford