9 replies to this topic

[chemiboy11]

here is a simple question:

Two pumps are connected in series. Two pumps are connected in parallel.
Which configuration would be more efficient? (discussing pressure drop, pump power and energy requirement)

For ease of understanding, consider the overall flow rate of both the systems to be equal, pumps are of exactly the same type, piping and installation costs are not necessary.

[fallah]

chemiboy11,

Two identical pumps when are connected in series and parallel to feed the same system, cannot deliver the same flowrate at all,.

Fallah

Edited by fallah, 10 April 2012 - 05:26 AM.

[kkala]

Chemiboy11 may mean that for a required flow Q and head H, what option of two pumps is more efficient:
2 pumps in series, each Q, H/2
2 pumps in parallel, each Q/2, H
Both options are theoretically equivalent to a single pump Q, H.
Can chemiboy11 clarify the query accordingly?
Both pairs absorb same shaft power, in the theoretical case when all four pumps have same efficiency on their operating point.

[chemiboy11]

by overall flow rate I mean to say that if the outlet of the pumps in series led to a tank, and that the two outlets of the pumps in parallel led to another similar tank, both the tanks would fill up at the same rate.

[Technical Bard]

The efficiency would depend on the specifics of each pump. It would be possible to select pumps in either scenario that were efficient.

The reason for using pumps in parallel is usually because the flow is too large for available pumps, or you want to limit the shaft power (so you can limit motor size and avoid very high voltages). The most common reason for parallel pumps is because you want to have a spare in case the single operating pump fails.

Pumps in series is generally not advised. This is because the maximum shutoff head of pumps in series is additive, and will result in higher design pressures in the downstream piping/equipment. There are cases where pumps in series are useful however, such as:

• Where there are equipment items that are cheaper at an intermediate pressure, such as heat exchangers or filters.
• Where the available NPSH on the suction cannot be achieved economically with a high head pump - therefore a low NPSH, low head pump is used as a primer for the high head pump.
• In slurry service, as there few multistage pumps manufactured for abrasive slurries. I have seen up to six pumps in series in mining slurry services.

[S.AHMAD]

by overall flow rate I mean to say that if the outlet of the pumps in series led to a tank, and that the two outlets of the pumps in parallel led to another similar tank, both the tanks would fill up at the same rate.

1. If all the pumps mentioned are of the same size and design, then obviously the one in parallel will fill-up the tanks faster.
2. We normally installed pump in parallel because we want higher total flowrate. Meaning, the volume of each pump is additive. The pump discharge pressure unchanged.
3. However, if we want higher pressure, that is one pump has insufficient pressure, then we install the two pump in series. The discharge pressure is additive, but both pump deliver the same flowrate.

[chemiboy11]

Now if we were to discuss the cost. The installation cost, piping cost, operating cost and the maintenance cost. Keeping the other things such as overall pressure of both the systems equal.
If my calculations show that I can use any of the two type of configurations for a required flow and pressure, which one should I choose as more cost effective?

[Dacs]

I tend to think low Q/high H pumps will be smaller than high Q/low H pumps.

[S.AHMAD]

Dear Chemboy
1. There is NO simple answer to you question, we need to analyze case-by-case in order to determine which option is more cost effective.
2. First of all, we must study the process requirements. Normally, the decision to install pump in series or parallel is DICTATED by process requirements rather than economics.
3. For example, for pumping of liquid through a furnace, it is safer to have two pumps in parallel so that anyone pump fails, the other pump is still running. This is to prevent dry tube leading to tube rupture in the furnace and to avoid costly plant shutdown (closing of ESV of fuel supply - last line of defense).
4. A case for requirement of pump in series is when we were upgrading crude unit capacity that is 4 pumps (have capacity but not enough pressure) are already installed. The choice is either to replace the the 4 pumps with new or adding booster pumps in series. In this case, the investment cost is cheaper to install two booster pumps (one spare)
5. In shorts, each pump arrangement has their own advantages and disadvantages. As a general guideline, pump in parallel is the preferred one due to lower operating risk (i.e the probability of failure is lower). Meaning, if only one pump fails, the other is still running. However, for pump in series, if one pump fails, the unit must be shutdown - operation stop that lead to significant loss of production.
6. Another example for pump in series is when the equipment (such as heat exchangers/pressure vessels downstream of the pump) design pressure is limited (lower) than the operating pressure of a single pump. This is normally for retrofitting cases.

Edited by S.AHMAD, 16 April 2012 - 09:47 PM.

[chemiboy11]

Thank you very much for your replies. I see your comments very informative. It will surely help me clear my views regarding the topic.