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Compressors And Silent |
The number one
silent root cause killer of compressors (both reciprocating and
centrifugal) in chemical plants, refineries, and gas processing plants is
inadequate knockout drum design. Knockout drums are also called
knockout pots, surge drums, separator tanks, separator pots, demister separators,
or gas cleaning equipment all serve the same purpose of gas-liquid separation.
The following compressor comments apply to large reciprocating and large
centrifugal compressors above 125 horsepower and above 35 psig pressures. It is difficult to prove the silent
root cause issue by objective evidence of target capture devices
installed inside the flow lines to the compressor, thus the issue must be
inferred from other experiences.
We know from long experience that steam turbine blades/vanes operating in
saturated steam are eroded by the impact of saturated steam (water)
particles. Wear resistant alloys are
needed on the leading edges of blades/vanes to slow down erosion while running
for long periods though a “rain shower”.
This says the size of particles is important.
We also know that small flying insects (lovebugs) in warm, moist climates during the summer
splatter automobile windshields and upon impact break into even smaller debris
that the coat automobile windshields and clog radiator intakes both problems
require immediate maintenance. The 1)
particle sizes and 2) flow of debris are both problems which require
maintenance solutions. Who solves the
particle size and flow issues for a compressor to prevent compressor failures?
In the medical field, a frequent finger pointing exercise exists among
doctors. The argument goes this way:
A cardiologist does not want his patient to die from a heart attack;
however the patient may die from kidney failure stemming from medicines given
by the cardiologist to prevent the heart failure.
So the finger pointing goes on in every field—it’s just not peculiar to
engineering!
In process plant designs chemical engineers are called upon to design the
process for flows. The chemical engineer
doesn’t want the process to fail, but often the lack of expertise in some areas
(such as knockout drum designs) often transfers the failure to a down stream
compressor because of inferior knockout drum design. The knockout drum becomes a silent root
cause of failure because of lack of understanding or insufficient
experience in the need for compressor flows to be absent of particles (both
solid and liquid). Mechanical engineers
are keen on gas streams free of particles both solid and liquid but the system
design has been set by the chemical engineers who lack experience in mechanical
details. So the finger pointing exercise
begins about who is responsible for compressor failures. So let’s set a design standard which will
provide long compressor life:
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The knockout
drum system must be capable of handling: |
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Items a) and b) are of direct interest for chemical engineers. Item c) must be planned and sized by chemical
engineers to protect the interest of mechanical engineers responsible for long,
failure free life of rotating equipment.
Cost of the properly specified and designed knockout drums incurs a one time
capital expenditure. However, failure of item c) is an ongoing expense which
disrupts production and incurs the high cost of lost profit opportunities which
will recur many times during the life of the compressor from the silent root
cause for failure.
The rough general relationship between particle sizes and equipment life is
shown in Figure 1 (of course individual compressor components make this
trendline very broad).
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Figure 1: Compressor Life vs Particle Size Ingested |
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A compressor can ingest an occasional large particle. It cannot sustain a torrent of large
particles. Some relationships between
ingested particle sizes and life have existed for many years. For example, the life of an automobile
internal combustion engine is ended after ingesting only one tablespoon of
dust/dirt! Bloch’s book on Reciprocating
Compressors on page 150-152 list four categories for foreign materials that
kill compressors and give clues of what to look for:
- Liquid carryover
(slug flow, carryover from interstage coolers, flow changes)
- Dirty gas
(solid particles act as grinding compounds and interference debris)
- Carbon formation
(high temperatures, oils, and gases make carbonaceous debris)
- Corrosive elements
(corrosion, erosion, and complex failure modes occur)
The hierarchy of simple to complicated devices for cleaning gas streams is:
- Parallel corrugated
plates are knockdown devices to eliminate large particles such as sand.
- Filters and bag houses
remove solid particles.
- Cyclones are ~90% efficient
for 40 micron particles or larger.
- Knockout drums,
without demisters, are ~90% efficient for 600 micron particles or larger
(these are OK for flare knockout drums but not for compressors).
- Knockout drums with cyclone(s)
build inside the drum will help eliminate fluids and solid particles ~90%
efficient for 40 micron particles or larger.
- Knockout drums with
thick and clean demister packs are ~98% efficient for droplets/particles
larger than 20 microns.
- Helical coil gas flow
separators remove 99.9% of all solids and liquid particles 6 microns or
greater which includes black powders occurring in natural gas pipelines
from the reaction of H2S, water, and iron in the pipe.
- Coalescing filters
handle liquids and solids with ~95% efficiency for droplets larger than
0.5 microns.
- Electrostatic filters
are good, but never as good as advertised, in eliminating small particles
because of maintenance difficulties of washing/cleaning.
- Polymer membranes are
coming on strong for selective removal of all size particles and for
selective removal of different fluids including separation of gases.
My experience says the silent root cause killer reasons
for compressor failures are one or more of the following reasons:
Insufficient knockout
capability in the original design without allowance for a 1.25 to 1.5 safety
factor. This safety factor is
consumed when flow stream creep occurs to overload the knockout drum.
- Design failures when knockout capability is
based on average flow rather than peak flow and without
calculations for slug flow capability.
Think of the newspaper headlines used in a well known statistics
class to emphasize the huge differences between the average and the
extremes:
“Man Drowns In River Of 6-Inches [15 cm] Average Depth.”
You don’t drown on the averages; you drown in the deep spots of a river! You don’t usually kill a compressor on the averages but extremes are dangerous as they transfer the particle size problem down stream to become a compressor failure from fluid particles/slug flow/solid particles in the flow stream.
- Design failure to eliminating dirt/moisture
particle sizes greater than 0.5 to 0.3 microns at 95 to 99%
efficiency. This is a stringent design criterion that would stop many
compressor failures.
- Design capability for flow surge must be
provided for handling occasional slug flow based on full line flow of
liquids for at least 5 seconds or longer periods of slug flow based on
knowledge of the peak problems.
- A common urban myth fallacy is that knockout
drums are believed to be such simple designs a draftsman can size
them. This myth will guarantee
downstream problems for the compressor.
- Another urban myth is to purposely heat the
process gas so you don’t have moisture particles in the gas in lieu of
other solutions. True, liquid
particles can be vaporized.
However, high temperatures add severe service onto the compressors,
and with limited long term success on highly loaded and sophisticated
compressors. This heating technique
is also reported downstream
of knockout drums and coalescing filters. The best solution is to eliminate the
liquid/solid particle problem at the source rather than generating a
secondary problem of higher heat in the compressor.
- A design flaw of ONLY using the Souders-Brown equation considers vapor velocity
but NOT particle size (see #3
and 4 above). Remember, the design
requirement for knockout drums is an AND solution for both
flow and escaping particle size, i.e., don’t exceed the design flow
and don’t exceed the maximum particle size. Chemical engineers, in
most cases, are only concerned with the average separation of liquids from
the inlet stream, but mechanical engineers are concerned with the particle
sizes and content of the particles in the gas stream which kills the
compressors. We need teamwork for achieving the investors lowest
long term cost of ownership.
(Yes, the issue is similar to the comment that the heart doctor
doesn’t want you to die from a heart attack but you may die from kidney
failure from the medicine he gives you for your heart.)
- Another urban myth is emergency relief flare
systems and compressors have the same requirements for knockout
drums. This is not true. Flare systems have are far less stringent requirements
(meaning they can handle larger particle sizes, 300 micron). Compressors have more severe
requirements than flare systems!!
Compressor genocide is practiced regularly in industry. To prevent compressor genocide you must
attack both the particle size and
the flow rate issues including provisions for slug flow. This infers different stages of separation
using some or all of steps 1-10 noted above.
Remember there are some cases where knockout
drums, demisters, and coalescing filters still have problems as described
at the Naval Air Station Lemoore. However, compressor manufactures also
stress the need for different stages of separation even for reciprocating
compressors to provide gas inlet filtering, liquid removal and liquid
coalescing systems upstream of the compressors.
Don’t be fooled. Your compressor failure
problems may simply be due to an up-stream problem with knockout drums (and
associated equipment to remove particles from the flow stream).
Heavily loaded compressors and sophisticated compressors have less tolerance
for handle insults from surges and particle sizes.
Treat your compressors like race horses. DO
NOT treat compressors like plug horses if you want to achieve high
reliability with long life between failures.
Remember, many compressor failures are self-induced failures. The silent root cause for many compressor failure stem from
knockout drums and other associated equipment for liquid and solid removal.
Knockout drum design has to be done correctly from the beginning. It’s
too expensive to make corrections after the error(s) has been proven. Furthermore in most facilities, no additional
space is available at a later date for installation of the correctly sized and
designed knockout equipment.
Further reading-
Amistco has a good paper on knockout drums including
use of a cyclone to knock down liquids (and solid particles) before reaching
demister pads and their double pocket vane units. They also provide some
design details including why you should anticipate sudden flow changes in
both directions. Amistco includes retrofit
solutions for existing designs.
Helical
coil separators from Mueller Environmental remove fluids, solids, and slug flow
from gas streams including pyrophoric black powder sulfides in pipelines with
99.9% efficiencies down to 6 microns which depends upon high velocity gas flows
to achieve the efficiency.
Coalescing filters allow small droplets in the gas stream to collide to
continuously enlarge as additional droplets collide and finally collect as they
move downward by gravity into the liquid collection system. They can be glass/plastic,
fine mesh knitted screens
for large particles, or sintered
metal for small particles.
King Tool produces a
self-cleaning, mist coalescing filter while the filter remains on line. The system periodically has a reverse flow to
each coalescing filter element to self-clean selected elements without a
shutdown. The reverse flow cleaning
extends filter life by a factor of 20-50 while trapping solid particles smaller
than one micron. Trapping and removal of
very small particles is helpful for removal of iron sulfide particles from sour
gas pipelines whereby the particles are flushed out in the liquid with minimal
re-entrainment.
Separation of amine from fuel gas is reported as a critical element in
elimination of burner tip fouling which requires the use of coalescing
filter devices as reported in NPRA Q&As.
Coalescing filters are very sensitive to flooding from too much fluid,
and efficiency drops rapidly when challenged with too much liquid whereby
liquid re-entrainment occurs as shown in the Pall
report of Figure 4. This means you need to oversize the
capability for coalescing filters and do not undersize the coalescing filters
to achieve long term success.
Other good design criteria are provided by the Iranian Ministry of Petroleum
for in Gas(Vapor)-Liquid_Separation and API
Spec 12J ( search for the 25 page spec and the price is US$94) for oil and
gas separators as devices located on the producing flow line between the
wellhead and pipeline. Routinely, slug
flow beyond the knockout drums are equipment killers.
The G(V)-L
Separation document calls for 2 to 5 seconds of normal feed velocity at 100%
liquid filling of the feed pipe. Most
folks go blind at the slug flow requirements.
They ignore surges. The designers
may have forgotten the surge requirement, but slug flow kills the compressors,
and damage due to the slug flow is cumulative on the compressors (it’s not just
a one time event).
Many knockout drums have the
demisters pads blown-out due to surges.
The result of blown-out demister pads allows 150 to 2000 micron size
particles moving downstream to the compressor as equipment killers. With demisters pads in place, the particle
sizes moving downstream may be 10 microns which are still too large for highly
loaded and sophisticated compressors to handle without damage.
The use of nanomaterials in coalescing filtration is
described in the WIPOA.
OK, so this is more than you wanted to know about knockout drums, coalescing
filters, etc. I’ve gathered these details to avoid the compressor
Achilles heel problem with chemical engineers designing knockout drums which
are incapable of handling slug flow as they satisfying their need for filling a
space on a PDF.
BUT lack of teamwork and
lack of understanding sets-up the mechanical engineers for failure on their
compressors because too many large particles (solid/liquid) are passed along to
kill the expensive rotating equipment for which chemical engineers have ~zero
appreciation for how susceptible compressors are to abuses of slug flow and too
many large particles entrained in the gas flows.
Where would you look for reference materials—
Look in Perry’s
Chemical Engineers’ Handbook, 8th edition for knockout drums and
you’ll find nothing in the index.
However, if you look in Section 6, Fluid and Particle Dynamics you’ll
find some details about falling liquid drops in gases. Section 10 pertains to Transport and Storage
of Fluids, Section 17 pertains to Gas-Solid Operations and Equipment, and
Section 18 pertains to Liquid-Solid Operations and Equipment—none of them
address knockout drums. The single
reference the chemical engineer would touch first is of no help for knockout
drums.
Ludwig’s
Applied Process Design for Chemical and Petrochemical Plants, 4th
edition is more helpful with 72 pages of illustrations and calculations for
mechanical separations involving:
1)
liquid
particles from vapor or gas,
2)
liquid
particles from immiscible liquid,
3)
dust
or solid particles from vapor or gas,
4)
solid
particles from liquid, and
5)
solid
particles from other solids.
In none of the chemical engineering separator cases does a design limit exist
for size of particle going into compressors [thus the criterion specified above
to satisfy both chemical engineers and mechanical engineers to solve expensive
failure problems together]. Of course
the particle size concept is implied by the class or grade of the separation
techniques but it is never clearly spoken for understanding by both chemical
and mechanical engineers. Take heed:
Figure 1 above defines a valuable criterion—in short, limit the size of
particles going into a compressor!
Some Final Words-
All separation systems are made for
selling. Some separation systems are
made for use with your compressors to achieve long life with few
failures. Don’t get caught in the leap
of faith with sweet sounding words by good salesmen. Use your head about what you want and
need. Be skeptical with show me, don’t
tell me the results to achieve long compressor life with few failures.
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March 8, 2010
© Barringer & Associates, Inc., 2008