Gordon,
Thank you very much for the interesting reading about
the practicl aspects of the pigs.
Keep mailing such useful tips.
Regards.
C. V. Gangadharan.
--- Gordon.Reddek@Alcan.com wrote:
> Yohanes,
>
> Sorry for the late reply. Below some of the most
> important issues to
> consider in launcher/ receiver design. The way I
> understand it drawings
> are not transferred on sites like this so here is a
> word picture of the
> issues I consider when embarking on a pig
> launcher/receiver design.
>
> The pig must fit loosely in the launcher/receiver so
> the barrel of the
> launcher/receiver must have a greater diameter than
> the pipe itself. When
> the pig is received the fluid has to pass around
> the pig in the barrel,
> so it would be a good idea to give the barrel a
> cross sectional area of
> about twice that of the linepipe, In your case say
> 28 inches. That will
> ensure that the velocity of the fluid passing around
> the pig is similar to
> the flow rate in the pipe when one or more pigs
> are in the barrel. You
> can make the barrel diameter smaller, however I
> would size it so that the
> flow velocity around the pig is less than 20m/s when
> flowing gas and 4m/s
> when flowing liquid. Be generous with barrel
> length. It is common for
> the barrel to hold a batch of pigs (say four or six
> pigs). Also, if you
> are going to use an intelligent pig on this line you
> will be surprised how
> long they can be. Find out what pigs you are going
> to use and make sure
> the barrel has adequate length.
> The reducer between the line and the barrel is not a
> trivial issue. I
> would use an eccentric reducer with the flat side on
> the bottom, at least
> on the launcher. A pig is easier to launch when the
> barrel and the
> pipeline bottoms are on the same level. If you use
> a concentric reducer
> it is difficult to raise the pig up at the end of
> the barrel and stick its
> nose into the line. Also, it sits there at and
> angle and sometimes jambs.
> If your launcher already has a concentric reducer
> then design a sliding
> carriage which you can use to locate the pig on the
> centre-line and slide
> it down the barrel into the mouth of the pipeline.
> This arrangement will
> also require a rammer to ram the pig into the mouth
> of the pipeline so
> that the carriage can be withdrawn. A sturdy wooden
> pole will do. Do not
> use aluminium in the sliding carriage and rammer
> because aluminium creates
> sparks when struck on rust.
> The barrel must be fitted with a closure and it is
> normal to install a
> quick opening closure. These are expensive pieces
> of equipment but make
> opening the barrel an easy task. I personally like
> the GD closure, most
> probably the most expensive around. The closure
> mechanism MUST have a
> means of ensuring that it can not be opened when
> their is pressure in the
> barrel. This normally takes the form of a small
> valve that accesses the
> barrel through the closure which must be open before
> the closure can be
> opened. It would be very dangerous indeed to open a
> closure when their is
> even a small pressure in the barrel so you MUST have
> a warning device.
> The launcher/receiver must have an isolation valve
> between the barrel and
> the pipeline. That valve MUST be a full bore valve
> to allow the pig to
> pass unrestricted into or out of the barrel. I am
> sure all of the valves
> on your pipeline will be full bore so that pigs can
> pass unrestricted
> through the pipeline. Also, it is very important
> that this valve have
> high quality sealing. It must be either a soft
> seated ball valve or a
> ball valve with metal to metal seating where the
> seats are ground and
> lapped-in to effect excellent sealing. This valve
> is the only barrier you
> have between the line contents and yourself when the
> closure is open, and
> if it leaks you will not only have fluid all over
> the show, but you may
> also find it impossible to open or close the closure
> because you can not
> get rid of the pressure or the flow.
> For the same reason, the barrel must have a good
> drain valve if it is a
> liquid system, or a good vent valve if it is a gas
> system. You need those
> to depressurise the barrel before opening it. Be
> generous with the
> sizing. It is very common for the barrel isolation
> valves to leak with
> time. Your colleagues may well need the good
> draining capability twenty
> years down the track.
> Each barrel is attached to the pipeline through a
> TEE. The line runs
> straight through the tee into the barrel, and the
> fluid is diverted around
> the barrel through the branch of the tee. Generally
> the tee is an equal
> tee however it is also common for the branch to
> reduce a size or two. The
> problem with this tee is that when a pig passing
> through it it could
> attempt to travel down the branch and not the
> straight run, so it could
> jamb in the tee. To prevent that ALL TEE's on a
> pipeline with a branch
> size more than say 40% of the pipe diameter are
> barred. Barring means
> that steel bars are welded across the branch of the
> tee flush with the
> inside of the pipe to guide the pig through the tee
> and to prevent it
> moving into the branch. There are a number of ways
> of doing this. The
> simplest and cheapest is to weld bars in there.
> That must be done with
> care to ensure that the welding does not affect the
> metallurgy of the tee
> or introduce cracks or imperfections. You may even
> have to heat treat the
> tee after welding. Another way is to weld a short
> stub of pipe onto the
> branch and to design plates that weld into that sub
> that protrude out
> into the tee where they act as bars. This whole
> assembly is heat treated
> after manufacture and then simply welded on the
> branch of the tee. Another
> way to do it is not to cut the line at the tee at
> all but to install a tee
> that wraps around the pipe. The branch of the tee
> is accessed by cutting
> slots in the pipe in the branch area.
> The tee must be located on the pipeline side of the
> barrel isolation valve
> and if it has a wall thickness greater than the
> linepipe, the inside
> diameter must be transitioned at a slope of about 1
> in 4 down to the
> linepipe inside diameter. If this were not done the
> ledge created between
> the two pipe sizes could jamb the pig.
> It is common practice to install a pig signaller
> (called a pig sig)
> downstream of the barrel isolation valve and the
> tee. This means that on
> the launcher the pig sig sits downstream of the tee
> and one the receiver
> it sits on the piece of pipe between the isolation
> valve and the reducer
> into the barrel. They are placed at that location
> because both the tee
> and the valve are capable of jamming the pig, so if
> the pig sig activates
> there is absolutely no doubt that the pig is either
> on its way or safely
> in the receiver. Make sure you put a short length
> of pipe between the
> reducer and the barrel isolation valve. That pipe
> gives space for the
> nose of the pig in a launcher, and is a good place
> to locate the pig sig
> in a receiver.
> The only other thing required is the bypass line
> around the barrel
> isolation valve. This is called the kicker line. It
> generally tees off
> from the branch line from the main tee I described
> above, and connects to
> the launcher or receiver barrel. You would not tee
> off the main line with
> the kicker line because that would require a second
> barred tee. It is
> not critical where it stabs into the barrel but it
> would seem a good idea
> to stab into the closure end of the launcher (to get
> behind a pig to push
> it) and into the reducer end of a receiver ( to push
> the
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