Alloy 20 liquid end with sulfuric acid 1

[MachinaMan]

Hi All,

I have a pump with an alloy 20 liquid end, the pump is used for a sulfuric acid application.

I am looking for a suitable material to pipe in the pump and avoid any galvanic corrosion.

I also need to use some existing carbon steel (A106) pipe so the material needs to be compatible with c/s also unless we use proper flange & gaskets which I prefer not to have in this application (source of leaks).

The material must be some form of metal, PVCs and plastics are out of the question.

Any suggestions and/or information sources

Thanks

RP

 

[moltenmetal]

PVDF-lined or Teflon-lined carbon steel? You can get it in 300# class if you're worried about leakage.

You imply that you need to mate existing CS to A20 somewhere in this system, either at the pump or in the piping somewhere- otherwise you'd just use A20 piping. There is a galvanic difference at this transition whatever way you slice it. The only way to eliminate that concern is via an electrically isolating flange break, which you imply that you don't want to use because you're worried about leakage. So you've over-specified the problem such that there's no solution. You have to figure out what you need and separate it from what you want.

Properly designed, an electrically-insulating flange break should be at low risk of leakage.

 

[MachinaMan]

Hi Moltenmetal,

Thanks for the reply.

I wasn't clear about the situation.

I need a transition piece between the alloy 20 liquid end of the pump and the existing carbon steel piping.

I need to eliminate any corrosion issues between the transition piece and the alloy 20 and also between the transition piece and the carbon steel existing piping.

Regards,

RP

 

[moltenmetal]

There are conc sulphuric acid experts here who can give you far better advice, but even a novice will note that LOW VELOCITY is extremely important to the corrosion resistance of CS or cast iron piping in conc acid service. So you not only need to get your liquid end connected to your piping, you need to get it up/down in diameter on the suction and discharge, and the velocity profile flattened out so there's no local high velocity, if your CS piping is going to survive.

Transition piping of PTFE- or PVDF-lined carbon steel would suit in terms of both corrosion resistance and electrical isolation to reduce galvanic worries, provided that proper isolation methods are used on the studs/nuts. Merely using fluoropolymer-coated studs and nuts will likely be insufficient.

 

[BillBirch]

What concentration are you using? This is very important for your selection.
Regards,
Bill

 

[MachinaMan]

Hi BillBirch,

We are using a 93% concentration.

Regards,

RP

 

[MachinaMan]

Hi Moltenmetal,

I think that you are refering to a flanged connection for the suction and discharge.
Our pump has female NPT connections for both, it's a small metering pump.

I like the lined C/S suggestion but our concern is that the C/S to alloy 20 threaded connection would cause a corrosion issue.

Thanks again,

 

[moltenmetal]

Unless the discharge side is at very high pressure, non-metallics (PFA tubing and fittings) would be your best solution as they provide both insulation and corrosion immunity rather than mere corrosion resistance. You'll need to safeguard these connections against mechanical damage. Armoured teflon hose with SS overbraid and A20 ends is available, as are A20 Swagelok fittings and rigid A20 tubing- but if you thread one of the male connectors directly into your existing CS piping you'll have a galvanic problem.

Even if you solve the galvanic problem properly, you may have a velocity problem at the connections between your small-bore suction/discharge lines and the existing carbon steel piping if your design does not take this carefully into account. Remember that a simplex PD metering pump puts out (and draws in) a pulsile flow with a peak flow of about pi times the average flow. You've got to worry about the resulting instantaneous high velocity this produces in the carbon steel piping. Velocities need to be LOW- 2 ft/s or so at most- to keep the passive layer on the CS parts, or else you'll be pumping them with your acid!

I hope your existing CS piping is NOT threaded for conc sulphuric acid! I'm a big fan of threaded piping- it's cheap to install, flexible, and with appropriate thread sealants it can be very reliable- but I draw the line for services like conc acid in CS material. Threaded is NOT appropriate for this service. A threaded joint to the head of your A20 pump? Sure, as long as the parts that thread into the pump are A20. Personally I'd transition to a flange there too unless tubing was the plan for the suciton and discharge connections to the pump.

The transition to the existing CS piping should be flanged. Fluoropolymer-lined flanged fittings or pipe spools can be used as a dielectric break between your CS parts and your new A20 parts if proper attention is paid to the use of electrically insulated bolting.

Best of luck to you.

 

[MachinaMan]

Hi moltenmetal

Our existing C/S piping has socket weld connections.
I'll look into the velocity, hose and flange connection as I make a decision.

Thanks for the help, star for you.