Clogging of DBB Valves - What solution ? 3

[DjazAutomation]

Hi,

In our gas processing plant and for many DP cell flow instrument, we frequently experience clogging of instrument DBB valves (double and Bleed valves)due to debris being present in the process fluid. Those are monoflange DBBs with three needle valves (two as block valves and one for venting).
The problem with the monoflange DBB is that the internal bore is really small and they tend to get easily clogged up by debris.

Ideally, a monoflange DBB designed for non-clean services would be the best solution but I couldn't find anything so far.

I have been told that ball DBB could be the answer since they can have a bigger bore (up to full bore). However there are concerns:
- Is the ball valve design suitable for non-clean services ? I mean is there a risk of loosing the seat sealing due to the debris? We might operate those DBB twice a year. Apart of that they are left open most of the time.
- On the HSE part, I have been told that the fact that the ball valve is quarter turn, the technician will not be able to slowly pressurize or depressurize the instrument which could lead to fluid hammer or damage to the instrument/tubing. I don't know if it's relevant since we are only talking about a flow transmitters and some tubing. don't know what's the normal practice is.

Any advice or recommendation will be highly appreciated

Thanks

Djaz

 

[LittleInch]

Please don't double post. You've been on this forum enough times to know it's not allowed.

Choose which forum you want then red flag the other post before someone else does...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

This forum makes the most sense, I've red flagged the other two.

I have to ask, "Why are you double block and bleeding an instrument line"? If there is some reason that you need positive energy isolation for an instrument line (I can't imagine one, but maybe one exists), it is trivial to shut a valve and remove a piece of tubing (you can even cap the process end if you are that nervous about it. You have a line that is a dead leg all year except twice when you calibrate. It is going to collect debris if there is any to collect. Any valve is going to be impacted by that debris. Using single-body DBB valves looks like you are just begging for problems without adding any offsetting value.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[LittleInch]

I'm struggling to work out where the gas flow is coming from that can cause this debris to clog the line. If you've got debris this implies heavy elements and normally flow.

Ball valve seals are susceptible to dirt, but if there is no flow and infrequent operation they should be OK.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

It is needle valves on a dead leg. I always see nasty junk in dead legs. A drop of water in a gas line starts growing a colony of anaerobic microbes, corrosion products get shifted from the main line into the dead leg by a change in the pressure in the line, garbage happens. It really doesn't take much junk to change the performance of a needle valve.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[DjazAutomation]

Thank you all for your replies. I really appreciate it.

The instruments where we have this issue are used for high pressure (75 Bar) and with fluids such as amine or hot oil. The EPC contractor installed those monoflange DBBs for energy isolation. The monoflange DBB are relatively small and light weight. That's why they are widely used for instrument isolation (especially for Hydrocarbon and chemical fluids, high pressure and high temperature).

All the Flow transmitters FTs installed in our plant are fitted with monoflange type DBB (needle valves). The disadvantage as I mentioned earlier is that those DBB tends to get clogged up frequently by debris.

Maybe I can express what I want to do as following: Is it okay to replace a needle valve with a ball valve to isolate an instrument (DP Cell FTs) and its impulse line which is a standard 1/2"npt OD tubing ? The seat sealing deterioration and the technician safety are concerns. The sealing deterioration will occur but the question is if it's acceptable considering that the valve will be operated twice a year.

I would appreciate any additional advise based on those clarifications.

Many thanks

Djaz

 

[LittleInch]

Is your monoflange (what size?) literally mounted on the flange of the tee or weldolet?

Are they all vertical?

DO you have room for a single isolating valve between flange and mono flonage? I realise that this takes away some of the advantages of the monoflange, but if you're having trouble then it may be required. This will help reduce the dirt able to get ot your MF, providing you're on the vertical side of the pipe.

Can you insert any filter or gauze in a spool between flange and monoflange? - something like this maybe?

http://www.lesker.com/newweb/flanges/hardware_kf_centeringrings_screen.cfm?pgid=0

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

LittleInch,
These are deadlegs so you don't have flow very often and you don't know which side of the strainer the problem is going to come from (most corrosion products will come from upstream but phase-change scale can come from either side). I've seen strainers plug in impulse lines, and the result is significant damping (the worst I ever saw required over 4 hours to register a 10 psi change). I try really hard to keep them out of systems I'm responsible for (we had one I&E tech that loved the things so all of my guys would let him leave location and shove a screwdriver through the screens).

Djaz,
I've used 1/2" steel-seated ball valves with really good results in high temperatures. Even the resilient seats can work well. I've never had a technician-safety issue with ball valves. Sometimes they leak. Sometimes needle valves leak. If I have high pressure/high temperature caustic or poisonous fluid then my procedures are going to always assume that the valves are going to leak and require adequate containment and PPE prior to starting work. If it is bad enough I'll fall back to my days in nuclear power and put the work in a glove bag and the worker in forced air. There is a way to do any job safely. You just have to find it.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[DjazAutomation]

Again, many thanks for both of you. Always good to see things from different perspectives.

Indeed, metal seated ball valves seem be to be a better fit for the application than the soft seated ones. I was looking for an inherently safe design (Full DBB) but it seems to me now that it's difficult to implement this solution (somehow over the top) given the space constraint and the increased load on the pipe nozzles (would require a strain analysis to be outsourced and probably additional supports).

As zdas04 mentioned,the right mitigation measures can always be put in place during the maintenance of those instruments. That's something I agree with and we actually do have an isolation procedure that gives the different additional measures if only a single or a double isolation are used. If I find a hard seated single ball valve with approximately the same weight, that would be perfect although I'll have to justify taking away the double isolation and vent feature of a DBB.

Upstream filtration is a good idea but I think that in our case, the filters would be frequently clogged up and we'd have to add an upstream isolation valve anyway to ensure isolation (more load on the nozzles). By the way, I didn't know that using strainer is an accepted practice in impulse lines. I wonder in which conditions it may be recommended.

Thank you very much for your support.

Djaz

 

[thost]

Hi Djaz

My selection for this application is usually a monoflange DBB valve with 2 metal seated ball valves in the main bore, and a needle valve for bleeding. I am also adding a instrument block and bleed manifold with needle valves to the transmitter pressure connection. This make it possible for the technican to slowly pressurize the instrument, but he will not be able to slowly pressurize the tubing if that is necessary. Normally, we expect that the tubing can withstand being pressurized immediately with 75 bar, but not the instrument.

This is a common way of doing this on oil installations in Norway. As long as my service is not extremely dirty, and the temperature is lower than 200C, this will be my first selection.

 

[DjazAutomation]

Many thanks thost

Do you know where I can get those monoflange DBB valve with 2 metal seated ball valves (suggested manufacturers if the Forum allow it) ? So far, I could find only resilient seated ones.
If this solution is adopted, the instrument maintenance procedure should describe clearly how the technician should pressurize the instrument (instrument manifold with needle valves).
The only constraint with this solution is the need to perform a stress calculation for all the nipoflanges we got since the new DBBs will be heavier than the existing ones. This will require to weld additional nozzle supports.
The solution is still valid and can be used where the following is not applicable:
- Use a small ball valve (metal seated) to avoid additional weight on the nozzle whenever the site isolation procedure allows it.
- Consider installing a close coupled instrument (no impulse lines at all!).

Any suggestions would be appreciated.

Djaz

 

[LittleInch]

The real issue here seems to be that you seem to have some long impulse lines which are allowing gas to flow in and out with the dirt / particles due to the gas in the impulse lines packing and de-packing. Hence as you note above mount the instrument on top of the mono flange - which is what I thought you had - and the issue should go away. If you can't then the ball valve mono flange is a way round it. Or just install a single valve flange on the flange as one half of your DBB and have the other half and a needle valve at the far end of the impulse line direct mounted onto the instrument.

I'm sure I've also seen diaphragm lines with liquid in them to transmit the pressure without allowing the gas into the impulse lines in similar situations. Something like this

http://wpa.ie/Products/PressureInstrumentation/DiaphragmSeals.aspx

It might be a bit more, but would probably make your problem go away.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[danw2]

Did you look at Hexvalves' Hexblok monoflange DBB with wide bore rising plug style valves?

Their flyer says "3/8" (10mm) orifice and porting means lower probability of plugging than all other competitive designs". They're metal seats, multi-turn operation like your people are accustomed to.

Page 20,

http://www.hexvalve.com/literature/NewLit/HexblokProduct.pdf