• Adsorption – The vessel is brought back on line in upflow to start the next cycle
by opening the feed and product valves.
There are pressure and time criteria for completion of each step before the
sequence will advance. If these are failed, the control system can adjust or, if
the failure is serious enough, it can shut down the PSA and block in the vessels.
Equipment Design and Metallurgy
Valves
PSA systems depend heavily on control valves to accomplish their function. Each
PSA vessel has four to six valves associated with it, depending on the number of
steps in the cycle. Each PSA vessel valve will go through around 35,000 cycles per
year on average. In addition, there are other valves that control surge pressures and
compositions of the off-gas or tail gas.
All of these valves must be routinely and regularly checked and serviced to
ensure reliability of the PSA system. Valve failure is the number one problem in
low PSA reliability.
PSA designers have specific valves they recommend for their units based on
extensive experience. It is wise to take advantage of their expertise. These are not
normal control valves. The valves have specific features tailored to the PSA service.
PSA Adsorbers and Cyclic Service
Just as each PSA vessel valve will go through about 35,000 cycles per year, each
vessel is pressurized and depressurized about 35,000 times per year. This cyclic
stress on a vessel can exacerbate small defects in construction of the vessel,
resulting in cracking of vessel.
This cyclic loading has to be recognized in specifying and monitoring the
vessels. Normally, there are periodic inspections for cracks growing from the cyclic
stresses. Inspection methods continue to develop for this service.
Off-Gas/Tail-Gas Surge Drum
The off-gas or tail-gas surge drum is a large vessel designed to moderate the
pressure and composition changes that come from the PSA vessel cycles. This is
required to provide a relatively stable fuel supply to the SMR furnace or other fuel
gas users.
At one time, there were two surge drums employed – one to moderate the
pressure cycles and a second drum to even out the compositions. This two-drum
system resulted in very stable off-gas flow and composition.
Recent practice for PSAs uses a single surge drum combined with improved
control valve logic and tuning to provide stable off-gas flow. This has not been quite
as effective as the two-drum system, but works well enough and significantly
reduces capital cost and footprint.
The surge drum is mostly just an empty space, but the inlet lines from the PSA
usually terminate inside the surge drum with elbows or distribution headers. These
Hydrogen Production and Management for Petroleum Processing
629
by opening the feed and product valves.
There are pressure and time criteria for completion of each step before the
sequence will advance. If these are failed, the control system can adjust or, if
the failure is serious enough, it can shut down the PSA and block in the vessels.
Equipment Design and Metallurgy
Valves
PSA systems depend heavily on control valves to accomplish their function. Each
PSA vessel has four to six valves associated with it, depending on the number of
steps in the cycle. Each PSA vessel valve will go through around 35,000 cycles per
year on average. In addition, there are other valves that control surge pressures and
compositions of the off-gas or tail gas.
All of these valves must be routinely and regularly checked and serviced to
ensure reliability of the PSA system. Valve failure is the number one problem in
low PSA reliability.
PSA designers have specific valves they recommend for their units based on
extensive experience. It is wise to take advantage of their expertise. These are not
normal control valves. The valves have specific features tailored to the PSA service.
PSA Adsorbers and Cyclic Service
Just as each PSA vessel valve will go through about 35,000 cycles per year, each
vessel is pressurized and depressurized about 35,000 times per year. This cyclic
stress on a vessel can exacerbate small defects in construction of the vessel,
resulting in cracking of vessel.
This cyclic loading has to be recognized in specifying and monitoring the
vessels. Normally, there are periodic inspections for cracks growing from the cyclic
stresses. Inspection methods continue to develop for this service.
Off-Gas/Tail-Gas Surge Drum
The off-gas or tail-gas surge drum is a large vessel designed to moderate the
pressure and composition changes that come from the PSA vessel cycles. This is
required to provide a relatively stable fuel supply to the SMR furnace or other fuel
gas users.
At one time, there were two surge drums employed – one to moderate the
pressure cycles and a second drum to even out the compositions. This two-drum
system resulted in very stable off-gas flow and composition.
Recent practice for PSAs uses a single surge drum combined with improved
control valve logic and tuning to provide stable off-gas flow. This has not been quite
as effective as the two-drum system, but works well enough and significantly
reduces capital cost and footprint.
The surge drum is mostly just an empty space, but the inlet lines from the PSA
usually terminate inside the surge drum with elbows or distribution headers. These
Hydrogen Production and Management for Petroleum Processing
629