Converting existing pumps to in series operation

[LTWayne]

I have an existing pump for naphtha service (aka Pump 2) with the following specs:

Suction P: 80.2 kPag rated, 86 kPag actual, 399.9 kPag max per spec sheet
Capacity: 178 m3/h rated
Diff head: 1164 kPa
Suction elevation: 0.78 m

The pump has a minimum flow spillback line controlled by an FIC right at the discharge of the pump (max 90 m3/h flow).

The pump has a S&T heat exchanger (hot side, dP at ~69 kPa) at the suction whose inlet and outlet piping rises to 9.6 meters and at that point the service fluid flashes due to the pressure being lower (120 kPag) than its vapor pressure at 167 kPag.

To solve this issue, I plan to route the naphtha first to another pump (aka Pump 1) to increase its pressure before routing this to the heat exchanger. The expected pressure at Pump 2 suction would be at ~224 kPag. The specs of Pump 1 are the following:

Suction P: 140 kPag rated, ~170 kPag actual
Capacity: 155.39 m3/h rated
Diff P: 137 kPa
Shutoff diff P: 180 kPa
Suction elevation: 0.53 m

Are there any other considerations that I need to check? What could possibly go wrong with this kind of setup?

Thanks!

 

[EdStainless]

A lot of things could go wrong, you could exceed the max inlet P on one of the pumps.
But more likely the issue will be do the pumps run anywhere near BEP?
Lay out these pressures and flows (based on actual flow and pressure drop data) and see if they actually fall on the pump curves.
Remember to correct for density, if you have much temperature change the densities will be different for each pump.
You may need some control valves in order to balance the system.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[LittleInch]

There's a lot of data there but difficult to follow without a schematic / P&ID and an elevation sketch.

Things to consider are:

Max inlet pressure at no flow into pump 2
New downstream pressure from P2

How you plan to control the flowrate / pressure into P2. You really will need to control flow in P2 on inlet pressure or have that set at some sort of minimum as well as flow.
How you will start these pumps / pump sequence / delay in starting P2 to allow pressure tor ise
Coordinated shutdown / trip
Check what the seals are in P2 and what they are rated for as a continuous pressure.

These seem very low pressures though.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

The max shutoff pressure in the series set up appears to be 170+180= 350kpag, but this hasnt accounted for static head of liquid in the tower bottoms sump. This is very close to the max permitted 400kpag suction pressure for existing P2. Presume your FIC control point at 90m3/hr prevents approaching this pressure. Would suggest a high pressure trip at 390kpag or so at P2 suction in case some one tampers with FIC setpoint at the control room. Also work out max suction pressure at P2 for worst case high density at low operating temp and what you would do for cold start also ( a startup P1 manual bypass perhaps?).
If max permissible 400kpag to P2 is exceeded in some cases, then a PIC at P2 suction or PIC - VFD at P1 may be required.

How does this planned increase in pressure affect preheat HX performance, given that vaporisation of the exit stream is now not possible? Any mechanical design pressure concerns here? Process safety concerns with tube rupture events also should be accounted for.

Agreed, check the suitability of the shaft seals on P2 and P1 also for this new service.

 

[LTWayne]

Thank you everyone for your replies. I now understand the need for additional controls at Pump 2.

Our current plan is to add a PI at Pump 2 suction linked to a safety interlock that will trip Pump 1 at low suction pressure. As for the HX performance, the MAWP is more than 2x the max shutoff pressure so it should be able the additional pressure from Pump 1. We will also check if the shaft seals are acceptable for the new service.