Psv fire training weldfit

Fire Case PSV Sizing Common Mistakes
Presenter: Eric Parvin, Parv Consulting
Hosted by: Adam Murray, Weldfit

PSV Evaluations
Very brief review of our company:
We do much more than PSVs-but that is one specialty. We are voting members of API 520/521, contributing
editor of GPSA Engineering Data Book 14th ed., former adjunct professor at Colorado School of Mines, and a
PE in many US states and P.Eng. in Alberta, Canada.
We offer training in PSV’s, distillation tray design, and overall fractionation train designs.
Our process engineering expertise ranges from upstream to refineries and chemical plants, and is extensive
in equipment design, troubleshooting and more. See our website for details: www.parvconsulting.com.
We also host a “Process Pop Quiz” on LinkedIn from time to time. See here for past articles and pop quizzes
at: https://www.linkedin.com/in/eric-parvin-p-e-p-eng-b2519b29/
Current Customers:
Syncrude, Suncor, Big West Oil, Consumer Energy, Hellervik Midstream, Encana, Keyera, Halker, Southcross,
Weldfit, and more.

Why the Class?
Lots of errors found when performing independent audits of others
• Software Improvements in Industry
• Hysys Dynamics / Safety Analysis Tool
• VMG PSV Sizing
• Many other PSV software now available (iPRSM, PSPPM, others)
I encourage other EPC firms to join our class—drives unity and consistency among
the EPC’s.
Don’t Want “3 Engineers, 4 Opinions”—nobody wins.
Because of this happening with one client, we have developed a PSV guideline on
how to address all scenarios, especially fire sizing.
3

PSV’s Don’t Prevent…
Recent event: Port Neches, TX explosion
Fire went on for ~ 12 hour before the tower in the picture below
”took off”. Please remember—PSV’s prevent overpressure, they do
not prevent over-temperature. Normal carbon steel vessels will
typically yield at lower pressures when temperatures exceed 800-
1000F.
Initial explosion:
https://www.youtube.com/watch?v=yosSaBMHLKc
Secondary explosion (the vessel that launches is a distillation
tower)
https://www.youtube.com/watch?v=FBh8ahDa04Y
And
https://www.youtube.com/watch?v=18WZaFGPpoM
The afternoon explosion, people >52 miles away said the walls in
their homes shook.
Copyright: Parv Consulting
(Eric Parvin / Melissa Parvin) 2016
4

Fire Cases Normally Apply
Can I dismiss fire?
ASME VIII Div. 1 UG-125-UG-140 discusses this in sections…in short
• If no flammables are around—you can:
• Must have appropriate multi-discipline sign-off
• Other requirements listed in ASME code
• If flammables are around, then need to have a PSV sized for fire case—period.
• Seen several upstream clients “dismiss” fire as not needing to size it
• Fabricators for heater treaters or other upstream common vessels provide a nozzle
TOO SMALL for appropriate PSV (NOTE: fabricators are not necessarily responsible
for sizing this…analogous to internal flange gasket).
• If you’re in a code compliant state, ASME must be followed.
• If you’re in a non-code state, but the vessel is ASME stamped, check your state’s
requirements—interpretations may still mandate following ASME for PSV sizing if
it’s built to ASME code even though the state is not a code compliant state.
5

Fire—For Today’s Discussion
Pool Fire (Large Liquid Pool) of Refinery Liquids.
6
FOR THE HEAT FLUX INTO THE VESSEL
Familiar Equation
Q = C * F * Aws 0.82
C = 21000 or 34500
F = Environmental Factor (e.g. fireproof insulation)
Aws = Wetted Surface Area (ft2)—area inside the vessel where liquid absorbs heat
Most commonly see Q= 21000 * Aws0.82
There’s a lot of assumptions in that 21000 number….
Won’t discuss insulation today

Fire—C coefficient
The 21,000 coefficient assumes
• Good Drainage
AND
• Good firefighting techniques.
Right—typical well pad in remote Weld County, CO.
Below—typical refinery.
7

Fire—Wetted Area
Wetted Area factor is dependent on the liquid level assumed inside.
We have followed behind many other larger EPC firms and PSV
specialists that use high liquid level (HLL) or a conservative value,
and report a PSV as being to small.
What level do you use?
• Should be only the area within 25 ft. of “local grade” (solid surface, including elevated
decking with solid flooring)—see 4.4.13.2.2 for details as to why.
• See API 521 4.4.13.2.3—provides the liquid level to use in most vessel applications
• For example, towers, NORMAL liquid level plus height of liquid on all trays above it
• For working storage (reflux drums, feed drums), use maximum liquid level
• Other examples exist in API
• 4.4.13.2.2 states that vessel heads inside skirts can be NEGLECTED from wetted area
(MANY contractors don’t take credit for this)
• Connected piping should be considered—(not detailed today)
8

Fire—Wetted Area
Q = C * F * Aws 0.82
Rarely noticed is API 521 4.4.13.3…if the fire is in a confined area,
then the heat flux can be higher, and this section of API 521 states
that the 0.82 power should be changed to 1.0.
Potential examples: Inside compressor houses; inside specialty
chemical housing areas; heater treaters inside sheds for
winterization; air compressor buildings (which have flammables in
there too); and anywhere there’s a building, shed, or confining space
for a fire to occur. A vessel against a wall is also to be considered.
9

Fire—Wetted Area
What about when a single PSV protects multiple vessels?
The wetted area of 3 vessels is shown below. Assume the same liquid is in each vessel for
ease of comparison (e.g. latent heat = 200 BTU/lb).
Q= 21,000 * (A1+A2+A3)^0.82 or 21,000*(A1^0.82 + A2^0.82 + A3^0.82) ?
Area 1 = 100 ft2
Area 2 = 200 ft2
Area 3 = 300 ft2
Group 1, total Q = _3.98MMBTU/hr_ or Group 2, total Q = __4.79MMBTU/hr_
Difference of 20%. The correct way is Group 2.
API has commented on this previously, but also remember the equations were derived
empirically using single vessels. Also “simple math” tells you this is true.
Now we have a heat flux—what next?
10

Fire—Latent Heat
Once you have the wetted area you can find the mass flow rate for the relief scenario through
the PSV by m = Q / λ (instead of λ we will use “LH” from now on)
What is latent heat?
• At normal (below supercritical) conditions, it’s interpreted by most as the energy to vaporize
1 lb. of liquid into vapor.
• From a chemical (single component) perspective, it’s the change in enthalpy between 1 lb of
liquid and 1 lb of vapor formed
• Is determined AT RELIEVING CONDITIONS--Can be at full accumulation pressure
• For fire cases—it’s the energy required to ”liberate” one pound of material from the liquid.
11

Fire—Latent Heat
Where does API discuss latent heat?
API 521 4.4.13.2.5.2
Also:
“On occasion, a multicomponent liquid can be heated at a pressure and temperature that
exceed the critical temperature or pressure for one or more of the individual components.
For example, vapors that are physically or chemically bound in solution can be liberated
from the liquid upon heating. This is not a standard latent-heating effect but is more
properly termed degassing or dissolution. Vapor generation is determined by the rate of
change in equilibrium caused by increasing temperature.” (italics added)
For these and other multicomponent mixtures that have a wide boiling range, it might be
necessary to develop a time dependent model where the total heat input to the vessel not
only causes vaporization but also raises the temperature of the remaining liquid, keeping it
at its boiling point.
12

Fire—Latent Heat
SENSIBLE HEAT: when boiling a fluid (let’s HD-5 propane) from bubble point to vapor, what’s
the enthalpy equation look like:
Q = m * Cp * dT + m * LH
The first term is the sensible heat portion of heating the liquid while simultaneously boiling
liquid into a vapor (latent heat, LH).
13

Fire—Latent Heat
Q = m * Cp * dT + m * LH
For multi-component systems that begin to boil, does this term exist?
Sensible heat does “exist” when boiling large boiling point range materials—
in fact it’s quite significant. It makes your LH for boiling off 1 pound of fluid
at relief conditions 10%+ higher.
Call this “Latent Heat with sensible heat effects”
(Hysys graph
Of the 2 curves)
14

Fire—Latent Heat
For multi-component systems or wide boiling point ranges, where do you evaluate LH?
Some rules of thumb to recall:
• As T increases, PSV area required increases
• As T rises, LH reduces, PSV area required increases more
• As T rises, MW increases, PSV area required decreases slightly
• As T rises, you are boiling off liquids, wetted area decreases, heat input decreases, PSV
area required decreases.
So let’s take a look at the competing variables in a vertical cylindrical
vessel (QUALITATIVELY, not a real example)
A=
W TZ / M( )
1
2
CKd
P1
Kb
15

Fire—Latent Heat
Look at % boiled vs. Heat Input
Vertical Cylindrical Pressure Vessel
0
200000
400000
600000
800000
1000000
0
20
40
60
80
100
120
0% 20% 40% 60% 80% 100% 120%
Wetted Area
Q
16

Fire—Latent Heat
Look at % boiled vs. Heat Input (Vertical Drum)
Simultaneously look at LH and Relief Rate Required
0
1000
2000
3000
4000
5000
6000
7000
0
20
40
60
80
100
120
140
160
180
0% 20% 40% 60% 80% 100% 120%
Lambda (assumed)
Relief Rate
17

Fire—Latent Heat
Working through a pseudo relief case, you can develop an area required graph like this one.
Notice the dip around the 10% mark and then localized peak in area required.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0% 20% 40% 60% 80% 100% 120%
PSV Area Required
Area Required
18

Fire—Latent Heat
Have seen similar systems (esp. In horizontal drums that start nearly liquid full) these other
colored curves for area required first-hand in various systems, especially crude oil, gasoline,
diesel, and other wide boiling point range hydrocarbon mixtures.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0% 20% 40% 60% 80% 100% 120%
PSV Area Required
Area Required
19

Fire—Latent Heat
Before we look at an example of latent heats, let’s discuss popular methods
to extract latent heat from a simulator:
A) Liquid “Properties” tab
B) Vapor Condensing
C) “100 drums, 100 latent heats”…(not shown)
D) Set up a “case study” with 1 drum—”True Latent Heat” below
20

Fire—Latent Heat
One example with methane concentration:
21

Fire—Latent Heat
Do the case study across the entire boiling range:
22

Fire—Latent Heat
Gasoline range material (e.g. LSR Naphtha)
23

Fire—Latent Heat
Gasoline range material (e.g. LSR Naphtha)—case study across all Boiling
Ranges
Anywhere from 15%-21% difference
between Method D and Method A
24

Fire—Latent Heat
So where have we seen SIGNIFICANT differences in latent heat values when
accounting for sensible heat effects?
• Hydrogen solubility in liquids (hydrotreaters, reformers)
• Seen LHE/SE values in the 2000 BTU/lb range at 10% boiloff point, vs.
150-200 simple simulation LH values.
• Methane solubility in liquids
• Seen values in the 800-2000 BTU/lb range…
• Ethane solubility
• Not as significant as methane, but still increases significantly in
heavier oils (e.g. not in LPG streams)
25

Fire—Software Options
How do you review this detail yourself? A few options exist…
Hysys has a “Safety Analysis” tool. You can program a PSV.
SO LONG AS YOU DON’T CROSS THE SUPER-CRITICAL RANGE (Hysys defaults
it at z<0.8), you can run a “Semi-Dynamic” analysis.
26

Fire—Time Dependent Analysis
Click on “Edit Flash Table” and you’ll see this…
27

Fire—PSV Software Options
PSPPM also has a good version of doing the same thing as Hysys.
We have an approach to PSPPM, iPRSM, Hysys, and a few other PSV
software calculation tools to ensure the correct values are used to give the
overall PSV area required on the time dependent analysis.
Salus also has a good tool for doing semi-dynamic analysis similar to Hysys.
VMG can perform a more rigorous fully dynamic analysis to flare systems
and is a good tool as well.
28

Fire—Other Factors to Consider
Fire—for liquid full vessels
A) In the 1990s it became popular to assume for liquid full vessels, you must assume 2 phase
relief through the PSV
B) Research later concluded that this is not necessarily needed so long as the PSV inlet line is
at “a high point” from the source of vapor generation.
If the PSV is installed at a low point in the vessel (e.g. on the bottom piping of a heat
exchanger), then you “should” design the PSV to relieve the same volumetric expansion of
vapor generation but as liquid expulsion. (Notice it says “should”, not “shall”)
See API 521 Section 4.4.13.2.5.3 for details (3rd paragraph).
Examples:
• Liquid filled filters with PSV coming off side of filter
• PSVs protecting heat exchangers with PSV off bottom of exchanger piping (seen it)
29

Fire Case Heat Flux Checklist
 Is this a pool fire case? – If no, other methods are necessary.
 All calculations are assumed to be blocked in vessel fires
Q=21,000 x F x A0.82
 Prompt and effective firefighting and firefighting equipment are available to the area (if not, change 21,000 to
34,500)
 Good drainage in the area. (if no, change 21,000 to 34,500)
If taking credit for insulation
 Has it been field verified as installed properly and materials of construction of jacketing and banding verified
to withstand fire temperatures?
 Has the fill material & thickness of insulation under the jacketing been verified?
Wetted area
 Correct liquid levels used, and elevation (25 ft. rule) applied properly?
 Skirt on vessel? (If yes, was area of skirt excluded?)
 Is it a confined fire? (if yes, change the 0.82 value to 1)
 Determine if any other exceptions to wetted area are applicable to the situation at hand.
 For protection of multiple vessels, fire variables calculated for each vessel independently
 Verify how the level transmitter is calibrated and spanned vs. PI data.
 Confirm the types of heads and presence of water boots
30

Q&A
Any Questions on Fire Cases?
We’ve only scratched the surface…

Thank you!
Thank you for your interest in the class and coming today.
If we can be of any service to you on PSVs, distillation, exchanger or
vessel design, or any operating issues you may have, please contact us.
Special thanks to:
Adam Murray, Weldfit

Psv fire training weldfit

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