1 reply to this topic

[mykid]

Hi I would like to seek for some advise /experience for Air operated diaphragm pump material selection.

Our client is looking to buy air operated diaphragm pump for chemical transfer on a FPSO. This will be a batch operation.
Some of these chemicals are classified as combustible liquid. The wetted part for the diaphragm pumps are is therefore specified as SS316. For the air compartment, vendor has offered polypropylene. This in general has no problem with the air side. However is this material commonly used in FPSO? GIven that it will be under sunlight all the time, it will wear off after few years.

My question is: is it necessary to change the air center block to SS316? The price difference and delivery is an issue here.

Another concern is if there is a fire, all the interconnecting pipe are SS316 so they can withstand to 540C minimum, but the air compartment as well as the diaphragm will melt first. This will cause chemical to flow out and adding as a fuel to the fire.

Since this is a batch operation, this scenario will only occur if fire breaks out during chemical transfer. These pumps will be elevated higher than the Tote tank.(suction lift: 1.2m) So strictly speaking, the instance where the PP section for the air part is gone, the transfer pump has lost its function, there will not be any chemical in the line. So could i considered that there is no safety issue here and therefore PP is acceptable?

I understood that there is no API standard for AOD pump. Is there any requirement for pumps that handle flammable or combustible liquid?

[kkala]

I will tell an opinion, despite my lack of experience on the matter. to be hopefully clarified by others.
1. In the past I worked in a fertilizer company, having installed (1978) air operated diaphragm pumps to transfer phosphoric acid - gypsum slurry from a thickener to phosphoric fertilizer unit. I assume that all was of 316 stainless steel (except diaphragm), due to corrosion of industrial phosphoric acid. Corrosion dictated 316 steel, combustible liquids (not containing corrosive compounds) can be transferred through carbon steel. Diaphragm was periodically replaced (after a period of few years).
2. PVC pipes exposed to sun were transferring phosphoric acid. We had not observed deterioration of them (their weak point was rather at the joints). Modern Polypropylene (of the air chamber) will most probably have the ability to withstand sunlight for a lot of years, supplier could advice approx replacement period.
3. So stainless steel as material of air chamber may not be necessary, but it is preferable. It has higher mechanical strength and higher temperature resistance (as you point out). Everything around seems to be of 316 steel (ss), so a welding between ss parts would be easier (and probably safer) than between polypropylene parts. Uniformity of material is an advantage.
4. Is it certain that pump and around will not contain (flammable) liquid during fire? I assume these pumps have "check valves" even in the side of liquid/slurry, thus keeping some liquid even after operation.
Diaphragm seems to be the critical element, which will be polypropylene at any case. I imagine that fire could "travel" to the tank through the open pipe; isolation valve at discharge could be a preventive measure.
5. I do not know API specific for this type of pump. Of course pump and motor should be suitable for the specific hazardous area, see API RP 500 or API RP 505.

Edited by kkala, 15 July 2011 - 08:08 AM.