Does anyone have experience on sizing the suction / discharge lines for such low capacity pumps? Or to be more precise, anyone sized chemical injection pumps with suction size of 0.5 inches?Magnetic pumps

You can forget about flanges. Yamada, Graco, and very other manufacturers have 1/4" ,3/8" & 1/2" ? NPT air operated diaphragm pumps, that can handle very low flow, and several fluids.

However, lets remember that this kind of pumps doesn't give you an uniform flow, but pulses of high-low flow. This can be avoided with a surge tank, but this will be an extra equipment you'll need to consider.

You may want to consider a metering pump or a peristaltic, (LMI Milton Roy is a manufacturer, I don't remember another right now).

The OP was positionned in the lower limit of classical multistage horizontal pumps and we declined. In fact it was needed a eccentric screw pumps to perform such dutySuber-water pumps.

Remember that this pumps operate different that centrifugal pump. You can adjust pump flow by adjusting inlet air pressure and flow (with the FRL).
If you need to change the flow head, you need to change the inlet air pressure. If you need to change the flow, you need to change the inlet air flow.

If you need to control the flow, you may need the surge tank and a control valve.

With a flow of 10gph [just under 0.2gpm], the flow velocity for a half inch pipe comes to about 0.28ft/s. This is far lower than recommended velocities, but as i was at pains to explain to my colleague yesterday, these recommendations (API 14E, for example) mention the words "if practicable"Screw pumps.

In the first case, for the heat medium system, the client expressly asked for a "small, pneumatic pump with a flow of 4 - 6gph". The original design was supposed to be by gravity flow to the make up tank, but the installers installed it at a lower elevation than the make up tank, hence my designing a pump. The pot is only 150mm diameter and 450mm height [2 gallons when full] and is filled manually, hence the low flow needed. I figure a design margin of 10gph pump should be adequate.

The second case, for the cooling water system, is similar (same size pot, same flawed installation with not enough static head) but they want to bypass the makeup tank and instead pump chemicals directly to the suction of the cooling water circulating pumps. The discharge pressure head is the pressure at the suction of these pumpsSelf-priming pumps.

If the fluid has/gives no problems flowing slow, let it.  Its going to be equally as hard making the suction line smaller than 1/2 inch.  The alternative is to dose the line for a few minutes at higher flow and shut it down until its time for the next dose.

It's a mistake to assume that 1/2" NPS is the smallest linesize available.

OD tubing and compression fittings are commonly used for linesizes lower than 1/2" NPS such as for metering applicationsChemical pumps.  With proper routing and support, they perform extremely well, for far less than the installed cost of pipe.

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