Tek-Tips is the largest IT community on the Internet today!
Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!
-
Congratulations cowski on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
Blow case sizing 2
- Thread starter moemantha
- Start date
-
1
- #2
I'm not aware of any rules of thumb for sizing, I'd think the primary thing to consider would be how often you want the valves to cycle in the run of a day vs. how much room you have to work with.
As far as tips, I'd say don't use one if you can avoid it - sometimes you don't have a choice though. I'm not sure what your intended installation is, I mainly see them in field compressor stations used to inject condensate knocked out of the gas stream back into the gas downstream of the compressor. Now, when set up properly, a blowcase will give you no problems whatsoever. If not, constant headaches. In the installations I see, they're usually set up with an inlet coming from an inlet separator dump valve. There's also a balance line from the blowcase into the compressor suction scrubber, so you have to make sure the pressure that initially drives the liquid (separator pressure, in this case) into the blowcase is higher than the pressure in the suction scrubber, usually at least 5 to 10 psi. This isn't too hard if you're only bringing in liquids from a single source upstream of the suction scrubber, but if you're dumping from multiple locations, it can get tricky. Also, unless the suction scrubber is at a higher elevation than the blowcase, there's no way to dump liquids from the vessel the blowcase is balanced to.
Drive gas is usually taken from the compressor discharge downstream of the aftercooler. When the level goes high in the blowcase, control valves on the inlet and balance line close, and a control valve on the power gas opens, blowing high pressure discharge gas into the blowcase. The blowcase outlet, either with a check valve or another control valve which opens, lets the power gas drive the liquids out. In my case, that means I have to introduce another 5 to 10 psi differential between where I take the power gas from my compressor discharge piping, and where I inject the condensate out of the blowcase back into it.
The differentials on both the suction and discharge can usually be introduced with a simple restriction orifice, if you have relatively stable pressures, or with a globe for either manual or automatic adjustment.
Problems with blowcases, first it can be tricky just synchronizing the four or more control valves required. They can cause a lot of problems during a shutdown as well. For instance, if a blowcase floods, it will flood liquids down the balance line into the compressor suction scrubber, which will usually bring the whole plant down. Also, depending on what kind of pressure differential you have between compressor suction and discharge, when the control valve opens to admit power gas you could actually freeze off your power gas line by the JT effect. Another problem is there's no way to drain the liquid from the vessel the blowcase is balanced to, as I mentioned before. There's also the added space required for a blowcase, the additional mechanical joints & leak points associated with all the extra piping and control valves (more critical in sour service than sweet), the need for a relief valve on the blowcase sized for fire case. Both the PSV and blowcase may have to be registered with a governing body, depending on where you are (I'm in Alberta, Canada, they'd have to be registered with Alberta Boilers here), and be inspected every few years.
Blowcases do have their purpose, especially in condensate services where the liquid can be sitting at it's vapour pressure, and when set up properly do work quite well, but you have to look at them from every possible angle first, and still expect some trouble-shooting after it's installed.
As far as tips, I'd say don't use one if you can avoid it - sometimes you don't have a choice though. I'm not sure what your intended installation is, I mainly see them in field compressor stations used to inject condensate knocked out of the gas stream back into the gas downstream of the compressor. Now, when set up properly, a blowcase will give you no problems whatsoever. If not, constant headaches. In the installations I see, they're usually set up with an inlet coming from an inlet separator dump valve. There's also a balance line from the blowcase into the compressor suction scrubber, so you have to make sure the pressure that initially drives the liquid (separator pressure, in this case) into the blowcase is higher than the pressure in the suction scrubber, usually at least 5 to 10 psi. This isn't too hard if you're only bringing in liquids from a single source upstream of the suction scrubber, but if you're dumping from multiple locations, it can get tricky. Also, unless the suction scrubber is at a higher elevation than the blowcase, there's no way to dump liquids from the vessel the blowcase is balanced to.
Drive gas is usually taken from the compressor discharge downstream of the aftercooler. When the level goes high in the blowcase, control valves on the inlet and balance line close, and a control valve on the power gas opens, blowing high pressure discharge gas into the blowcase. The blowcase outlet, either with a check valve or another control valve which opens, lets the power gas drive the liquids out. In my case, that means I have to introduce another 5 to 10 psi differential between where I take the power gas from my compressor discharge piping, and where I inject the condensate out of the blowcase back into it.
The differentials on both the suction and discharge can usually be introduced with a simple restriction orifice, if you have relatively stable pressures, or with a globe for either manual or automatic adjustment.
Problems with blowcases, first it can be tricky just synchronizing the four or more control valves required. They can cause a lot of problems during a shutdown as well. For instance, if a blowcase floods, it will flood liquids down the balance line into the compressor suction scrubber, which will usually bring the whole plant down. Also, depending on what kind of pressure differential you have between compressor suction and discharge, when the control valve opens to admit power gas you could actually freeze off your power gas line by the JT effect. Another problem is there's no way to drain the liquid from the vessel the blowcase is balanced to, as I mentioned before. There's also the added space required for a blowcase, the additional mechanical joints & leak points associated with all the extra piping and control valves (more critical in sour service than sweet), the need for a relief valve on the blowcase sized for fire case. Both the PSV and blowcase may have to be registered with a governing body, depending on where you are (I'm in Alberta, Canada, they'd have to be registered with Alberta Boilers here), and be inspected every few years.
Blowcases do have their purpose, especially in condensate services where the liquid can be sitting at it's vapour pressure, and when set up properly do work quite well, but you have to look at them from every possible angle first, and still expect some trouble-shooting after it's installed.
-
1
- #3
moemantha,
My experiences are WAY different from scipio's. I've got over 100 blowcases on gas wells and they have been working at least as well as the production separators for 7 years. Blowcases are my first choice for low pressure accumulations of liquid (my second choice is a Wilden pump and that is just a wasteful alternative).
The control systems are based on a single signal that shuts the balance valve then opens the power gas and the dump (an orifice in the line controls the sequencing). I'm taking water from the bottom of a 2 phase production unit at about 5 psig and dumping it using compressor discharge into a 100 psig water system with no problem.
Sizing is based on how much water you have to move and how fast you can empty the blowcase. I looked at commercial designs and found them all to be horizontal units that had to pressurize a large gas/liquid interface before they could dump - a slow process. I designed my own vertical unit (based around an 8-inch pipe and an 18" throw in a level controller - an ASME code vessel) that dumps almost 4 gallons of water in under 10 seconds. That is 210 dumps for 20 bbl/day, or less than 9 dumps/hour. With 15 seconds to fill and 10 seconds to empty I could move something like 325 bbl/day and have moved over 150 with no problem.
Several fab shops are starting to make integral blowcases that eliminate the piping and space problems that Scipio talks about, and the ones that are taller than they are wide work pretty well.
Hope this helps.
David
My experiences are WAY different from scipio's. I've got over 100 blowcases on gas wells and they have been working at least as well as the production separators for 7 years. Blowcases are my first choice for low pressure accumulations of liquid (my second choice is a Wilden pump and that is just a wasteful alternative).
The control systems are based on a single signal that shuts the balance valve then opens the power gas and the dump (an orifice in the line controls the sequencing). I'm taking water from the bottom of a 2 phase production unit at about 5 psig and dumping it using compressor discharge into a 100 psig water system with no problem.
Sizing is based on how much water you have to move and how fast you can empty the blowcase. I looked at commercial designs and found them all to be horizontal units that had to pressurize a large gas/liquid interface before they could dump - a slow process. I designed my own vertical unit (based around an 8-inch pipe and an 18" throw in a level controller - an ASME code vessel) that dumps almost 4 gallons of water in under 10 seconds. That is 210 dumps for 20 bbl/day, or less than 9 dumps/hour. With 15 seconds to fill and 10 seconds to empty I could move something like 325 bbl/day and have moved over 150 with no problem.
Several fab shops are starting to make integral blowcases that eliminate the piping and space problems that Scipio talks about, and the ones that are taller than they are wide work pretty well.
Hope this helps.
David
Actually we've got a few dozen blowcases operating on gas wells, and lot of them do work with the same reliability that zdas04 describes. It's just the few that don't want to work that cause repeated headaches - as described - if not thought out beforehand. All the blowcases here are horizontal, which may be part of the difference, and are usually dumping light hydrocarbon condensate (SG < 0.72), some of which is sour, and usually operate on compressors with 20 to 200 psig suction, discharge pressures upwards of 1200 psig. Way outside of Wilden territory.
I do agree if you're going to put one in, integral blowcases solve a lot of problems - some of my compressor packagers provide horizontal over-and-under configurations as options instead of suction scrubbers.
I do agree if you're going to put one in, integral blowcases solve a lot of problems - some of my compressor packagers provide horizontal over-and-under configurations as options instead of suction scrubbers.
- Status
Similar threads
- Locked
- Question