5 replies to this topic

[omnibus]

Hi all,

I am rating a sea water network. Can some good soul tell me or point out a source for, what should be the erosional velocity limit for FRP pipelines?

Thanks as always are in advance.

[Art Montemayor]

Omnibus:

Erosional velocity is an important subject when treating wellhead production development. API 14E is the specific document that deals with this subject and one that I believe you should make yourself familiar with.

I have dealt with this subject and constructed a spreadsheet that I included in my set of Fluid FLow Workbooks (3 in all). I uploaded these workbooks into a thread some time ago. I had to divide the many spreadsheets into 3 workbooks because one workbook was much too large to upload.

You should, in my opinion, NOT subject any FRP pipe to erosional effect. Plastic piping - whether reinforced or not - is NOT SUITED for handling any solids-contaminated liquid or gas stream, in my opinion.

[omnibus]

Thanks for the reply Art.

What I meant was if I am rating a network of sea water pipelines, what velocity should I consider as maximum limit for FRP pipelines?

I will look into API14E, but meanwhile any ready answers?

[Art Montemayor]

Omnibus:

Why do you persist in avoiding the real subject which you introduced and asked about - erosion of FRP pipe under a flow of seawater?

In your second post you again fail to state and identify with clarity exactly what it is that you are asking about. There can be no "ready answers" when you offer NO BASIC DATA.

Are you going to flow pure seawater? Or are you proposing to flow seawater with sand, stones, debris, etc? Please stop beating around the bush and tell us ALL OF THE BASIC DATA.

How can you expect a quick, correct, or reasonable answer to a request that is vague, general-at-best, and devoid of any basic data? If you starve us of basic data (flow rate, pressures, fluid composition, velocities, materials of construction, scope of work, length of pipeline, location, etc., etc.) we are eventually forced to starve you of answers.

I believe this is something like "quid pro quo".

[omnibus]

Hi Art,

There goes my self image of a good communicator in written text!

Thanks for taking the time to reply.

When I said 'Ready Answer' I never implied to undermine your effort. It was just as in case of other liquids we size or rate the (metallic) pipelines to keep the velocity within say 2-3.5 m/s to avoid high vibration and erosion.

The job involves a network of sea water pipelines, inside the plant ( for cooling purpose - so clean sea water), connected with eight Plate Heat Exchagers. Out of eight exchangers, seven are identical and eighth is of higher capacity and has FRP pipelines.

The problem is of high vibrations leading to failure of FRP piping at the time of backflushing the strainer in the inlet line to exchanger. The back flushing line is of smaller size (6") and a hydraulic analysis of design indicated very high velocity ( 19 m/s).

The solution I proposed is to provide ROs in all the back flush lines to keep the flow velocity within limit. RO will be sized based on the design flow of the Strainer during backwash, and rest of the flow at that time will go through the normal route, i.e. through exchanger.

The client also want to change all the lines to FRP lines with Exchagers with higher capacity due to one debottlenecking project. The increased flow ( in future) necessisated rating/resizing of the lines.

Now line size rating/resizing depends on what I consider a safe velocity for Sea Water/FRP Line combination. So what maximum velocity to consider as safe?

Hope I have made myself clear.

Regards,

O

[Art Montemayor]

Om:

Thank you for replying with the data and scope that you now furnish. We now have something we can sink our teeth into and, hopefully, contribute some worth to your project.

First, and foremost, I would advise you that this is a classical “trade off” type of application. The choice of FRP (Fiberglass Reinforced Plastic) pipe is, in my opinion, the proper way to go with seawater (as long as you stick very religiously to the fabricator’s recommendations and experiences in this application). However, not all FRP pipe is equal. Be very careful of the fabrication methods used, the bonding, the overlaying, etc., etc.. The best (and surest) way to know that you have a good Material of Construction (MOC) in the FRP selection is to always refer to tested, witnessed, and certified prior field user’s experiences with the same, identical MOC and the same identical seawater application. Do not generalize. I repeat once more for your attention: not all FRP pipe is equal.

The immediate trade off you confront is that although you may have found the perfect MOC, the application requires more of it. What I refer to here is that the FRP pipe schedule is very weak with regards to wall thickness and material bulk. The “welding” (glueing, actually) is of a weaker nature when compared to metallic fusion. The fabrication techniques employed are extremely important because of the possibility of defects in any one piece. Quality control and inspection have to be increased. The end result of the trade off effect is that you require more pipe supports, pipe anchors, slower velocities (3.5 m/s is too high, in my opinion), and possibly more expansion joints (always check out the MOC expansion coefficient and its effect). These EXTRA COST trade offs are to be expected. Nothing in life is easy or comes free. With less corrosion effect, you pay in another way. The client is often naïve and thinks that additional costs should not be tolerated. They are ignorant of the always-present trade off effect and must be forced to face reality or confront a possible bad application. This is your job to convince the client of what is really needed to ensure a good application. If you have an ignorant client, you have your hands full.

Keep your velocities low (preferably 5 ft/s or lower) and your pressure “shocks” down to nil. FRP piping is not as tough as steel piping and can’t take pressure hammering. Avoid as many fittings as you can. Every change in direction means a pressure drop and a potential for a water hammer effect.

I don’t know the scope or reason for the “backflushing” or the use of the resistance orifice (RO), but any sudden pressure change is a potential for a water hammer.

What maximum velocity is considered as safe? I would not proceed with the design of the system without a complete, written agreement on the type and make of FRP as well as the design velocities between you, the selected FRP fabricator, and the Client. In my opinion, this is a very important step in this type of project. You are not the expert in the FRP application. The more you embrace and stick by this reality, the better off you (& your client) will be. There are too many differing interests at stake in this type of application. The intent is good; the results can be disastrous. The Client wants a non-corrosive, cheap, and perfect installation that requires no maintenance; the Fabricator wants to sell as much product as possible - and that the installation be a successful showcase for future sales. You must take a role of complete, honest, and professional engineering. The FRP expert is the fabricator – logic alone determines this fact. If you proceed with a design of your own (without fabricator’s approval or backing) you are incurring all the responsibility with a minimum of the knowledge regarding the product at hand. Professional engineers are supposed to be a lot smarter than that. Use all the available resources wisely and correctly. Unless you have a background of FRP pipe installations, you are only a LEARNER for now.

I hope this experience helps you out.