Rotary lobe pumps installation

Possibly because you can only suck at atmospheric pressure* (whereas you can pump at several atmospheres with no problem) and thus there is a risk of one infufficient flow in the suction line, unless it is big diameter or you parallell them up.
Del
*Many people just don't realise this fact.

It took me a while to work out what you were saying here, it could have been better phrased. I guess you are saying that a pump cannot create more than one atmosphere in suction lift (in fact it will always be less, not even a commercial vacuum pump can achieve perfect vacuum), but two pumps in parallel will not necessarily create any more suction than one pump, and if the pumps are equal and the suction system is designed to provide enough suction pressure for the overall duty, then there should be no problem. In fact, all pumps are not created equal, and due to manufacturing tolerances it is very likely that one pump will perform better than the other. So the problem (and the reason for avoiding common suction) is that the stronger pump can starve the suction of the weaker pump, which could lead to cavitation if suction pressures are already borderline.

so conclusion of the discussion is that a larger diameter header (from the suction line diameter) is necessary for pump suction. Am i right?

That would certainly help, but the measures you need to take, and the degree of isolation of one pump suction from the other, will ultimately depend on the suction pressure you have at the design flowrate, and in particular the margin of NPSH available vs. NPSH required. If the suction conditions are very good then a larger diameter header would probably be enough. If NPSHa is already very close to NPSHr then you might have to consider seperate suction lines right back to the vessel you are drawing from.

One question: why do you want to put two pumps in parallel? If it is to double the throughput in an existing system that is not designed for such flowrates, you might find it will create pressures (both suction and discharge) that the pumps can't handle. In particular, make sure the suction pipework is adequately sized for the overall flowrate you require to avoid excessive friction losses.

"it could have been better phrased".
I think it's accurate and lucid, it's also briefer than yours.
The errors you apparently perceive in my post are due to you jumping to conclusions rather simply reading it word for word.

"I guess you are saying that a pump cannot create more than one atmosphere in suction lift "
No need to 'guess', that's exactly what I say.

"but two pumps in parallel will not necessarily create any more suction than one pump"
Indeed, and I don't say they will.
I merely talk of flow restriction.

I'm afraid your comprehension test score is 4/10, must try harder (just teasing)
Del
(BTW. Please note the first word of my initial post is "possibly")

I won't get into a cat fight over this, Del, I'm sure your claws are sharper than mine. In fact, I thought I'd done rather well to decipher your meaning given that I have a stinking cold and a fuzzy head.

I do take issue with your phrase "can only suck at atmospheric pressure" because it could have two meanings, neither of which are true, and both of which create non-sequitors of the remainder of the sentence. It could mean:

"can only suck when the suction line is at atmospheric pressure", which we know is not true. Or:

"can only create one atmosphere of suction pressure, and nothing else".

The pump will only create the pressures, both suction and discharge, that the system demands.

Indeed, ok my wording is amphibious, ambidexterous, ambivalent ambiguous.
We'll call it an honourable draw.
Hope your cold clears up soon.
I was just grumpy 'cos someone has just thrown away all the non production stock of resistors, without consulting the only person who could possibly ever need them ... me.
It wouldn't be so bad but this is the 3rd time they've throw away components and every time I say consult me...dunno why I bother.
Del (baaaad grumpy kitty)

Granted, and you have my sympathies. I bet that stock will have had just the resistor you'll be looking for in the morning.

Depending on number of lobes - flow pulse at a frequency

Can make harmonic - rob pump of suction. cavitate. rotor flap.

If can do, pressurise intake, then can do parallel pumps

Gear pump same but many teeth make much smaller pulses. make noise, not big damage

Just the off chance that if only one pump is running it could suck something into the other pump from wherever the other pump is normally pumping. In the case of a sewer treatment plant you could cause a real mess.

The running pump would soon be destroyed by whatever came backwards down the line. I feel that a lobe pump will motor, they spin freely when off. These pumps always have an air filter, and flow will seek the path of least resistance.