A local wastewater company asked for your help to solve the mystery of faulty operation in a pump station.

Problem statement: at random times pumping volumetric rates are reduced to such levels that wet chambers flood and wastewater overflows uncontrolled and some other times pumping rates come back to normal levels.

They delivered you the relevant drawings and technical studies and you take over the job to find out what is wrong. At first sight pump station consists of 6 parallel dry wastewater pumps, 2 wet collecting chambers and 3 parallel pressurized pipelines. Now how you go on?

Epanet model design

Load Epanet software and start to design the network skeleton holding in mind that you must:

• use the suitable epanet library objects
• keep the model light weight by transfering multiple local fittings on one vane with summarized friction coefficients
• draw the pressurized pipes from pump station vanes to the WWTP discharge tank 
• setup on each pipe a regulated valve in order to close / open it according to senario analyzed 

EPANET model parameters

After drawing the network on screen you must consider the right geometrical and operational parameter values:

• use large fittings friction coefficients from hydraulic literature
• select a method to solve your network (Hazen or Darcy)
• register the real pipe lengths, internal diameters and node altitudes
• summarize local fittings friction coefficients and assign them on a suitable local vane
• create the volumetric rate-pump head curve from the supplier literature
• create the volumetric rate-total efficiency curve from the supplier literature

EPANET model scenario run

To get physically meaning results by running a scenario you should assign first each scenario to a specific number of open pumps and pipes. You realize that there are too many scenarios to run. However there is a way to reduce this number. Pumps are similar and fittings installed on each line before reaching the pipes are almost the same. That means you should expect similar pumping characteristics from each pump running in a specific set of open pipes.

Abbreviating, for a group of n pumps you have to run 7 scenarios mentioned below.

where pl1, pl2 and pl3 the 3 parallel pipelines. 

Field investigation

Before announce your results you must detect which scenario describes best the problem encountered and which one gives the best solution. It is also a good practice to validate your results with real field measurements. You inform the company that data recordings must take place for a few days per minute or per a few seconds (head, power and volumetric rate). From problem description you realize that the most sensible change happens on volumetric rate. Increasing the vol rate increases the power, decreasing the vol rate decreases the power. You could use power measurements instead of volume rates in case there are no volumetric meters installed.

General

The wastewater pump installation type is dry, centrifugal with a curve vol rate-efficiency fit on a 2 or 3 polynomial degree and a maximum point (see diagram)

Pumps normally are sized at the maximum point or at a point of greater vol rate depending on how much space you want to allow for aging phenomena.

You eager to find out if that holds for your 6 pumps.

Seven scenarios-1 pump open

From the 7 scenarios analyzed with 1 pump open 3 of them regard 1 pipe open, 3 of them regard 2 pipes open and the last one regard 3 pipes open. You expect that greater total pipe cross section causes reduction to pump head and increase to total vol rate. The table below abbreviates the results for 7 scenarios. You selected wastewater pipe velocity and total pumping efficiency to show potential faults on operation.

Conclusions-1 pump open

What do you conclude?

• in all scenarios total pump efficiency and vol rate are far below the maximum point
• at maximum point eff is 71% and vol rate about 120l/s
• in the 1 scenario pump works with eff 36% at vol rate 4 times lower than the best point
• in the rest scenarios pump works with eff 61-64% at vol rate 2 times lower than the best point

It is obvious that the first fault in this system is: wrong pump sizing with high operational costs

Does this explain why the company faces the stated problem? By itself, not! But if you look at the table with velocities and compare them with a minimum acceptable velocity 0,5-0,6m/s (minimum velocity which scrapes the bottom of the pipe and leaves no sediment), then you realize the following:

• in the 3 first one pipe scenarios wastewater velocities are sufficient,
• the last 4 scenarios present specific problems
• in the 4 scenario pipe 1 velocity is lower than the minimum
• in the 5 scenario both pipe velocities are lower than the minimum
• in the 6 scenario both pipe velocities are lower than the minimum
• in the 7 scenario all 3 pipe velocities are lower than the minimum

What does it mean when a pipe fluid velocity is lower than the minimum acceptable? It means that there is a potential danger for pipe blockage because of gradual suspended solids sedimentation and we could imagine this as a pipe with a vane partially open-closed. What happens when a pipe blockage takes place?

• in scenario 4, if pipe 1 is blocked then fluid is served through pipe 2, in that case the system works as scenario 2, there is a chance for block breakage eg because of pump vibrations or hydraulic transients so the scenario 4 is restored
• in scenario 5, if pipe 1 is blocked then fluid is served through pipe 3, in that case the system works as scenario 3, if both pipes are blocked then there is a complete system failure and wastewater overflows the local area, there is a chance for block breakage eg because of pump vibrations or hydraulic transients so the scenario 5 is restored
• in scenario 6, if pipe 2 is blocked then fluid is served through pipe 3, in that case the system works as scenario 3, if both pipes are blocked then there is a complete system failure and wastewater overflows the local area, there is a chance for block breakage eg because of pump vibrations or hydraulic transients so the scenario 6 is restored
• in scenario 7, if pipe 1 is blocked then fluid is served through pipes 2+3, in that case the system works as scenario 6, if both pipes are blocked then there is a complete system failure and wastewater overflows the local area, there is a chance for block breakage eg because of pump vibrations or hydraulic transients so the scenario 7 is restored

It is obvious that the second fault in this system is: operational faults because of wrong pipes open when 1 pump is open

On the other hand, the thought "that only 1 pump open might not be suitable for each season" comes across your mind. In order to be sure you check the technical studies and discuss the issue with the company. No one can tell you for sure. You are willing to run 7 more scenarios with 2 pumps open, this way you can sleep better. You expect elevated velocities with 2 pumps open and you want to find out possible dangers.

Seven scenarios-2 pumps open

From the 7 scenarios analyzed with 2 pumps open 3 of them regard 1 pipe open, 3 of them regard 2 pipes open and the last one regard 3 pipes open. You expect that greater total pipe cross section causes reduction to pump head and increase to total vol rate. The table below abbreviates the results for 7 scenarios. You selected wastewater pipe velocity and total pumping efficiency to show potential faults on operation.

Conclusions-2 pumps open

What do you conclude?

• in all scenarios total pump efficiency and vol rate are far below the maximum point
• at maximum point eff is 71% and vol rate about 120l/s
• in the 1 scenario pump works with eff 23% at vol rate 8 times lower than the best point
• in the rest scenarios pump works with eff 56-62% at vol rate 2 times lower than the best point 

You also realize the following:

• in the 4 first scenarios wastewater velocities are sufficient,
• some of the last 3 scenarios present possible dangers
• in the 5 scenario pipe 1 velocitiy is close to the minimum and the pipe 2 sufficient
• in the 6 scenario both pipe velocities are close to the minimum
• in the 7 scenario pipe 1 velocity is close to the minimum and pipes 2+3 velocities are close to the minimum

What does it mean when a pipe fluid velocity is lower than the minimum acceptable? It means that there is a potential danger for pipe blockage because of gradual suspended solids sedimentation and we could imagine this as a pipe with a vane partially open-closed. What happens when a pipe blockage takes place?

• in scenario 5, if pipe 1 is blocked then fluid is served through pipe 3, in that case the system works as scenario 3, if both pipes are blocked then there is a complete system failure and wastewater overflows the local area, there is a chance for block breakage eg because of pump vibrations or hydraulic transients so the scenario 5 is restored, this scenario has better chances than the one with 1 pump because velocities are higher
• in scenario 6, if pipe 2 is blocked then fluid is served through pipe 3, in that case the system works as scenario 3, if both pipes are blocked then there is a complete system failure and wastewater overflows the local area, there is a chance for block breakage eg because of pump vibrations or hydraulic transients so the scenario 6 is restored, this scenario has better chances than the one with 1 pump because velocities are higher
• in scenario 7, if pipe 1 is blocked then fluid is served through pipes 2+3, in that case the system works as scenario 6, if all pipes are blocked then there is a complete system failure and wastewater overflows the local area, there is a chance for block breakage eg because of pump vibrations or hydraulic transients so the scenario 7 is restored, this scenario has better chances than the one with 1 pump because velocities are higher

It is obvious that the third fault in this system is: operational faults because of wrong pipes open when 2 pumps are open

Solutions with 1 or 2 pipes open

With recorded data available you may have the tools to judge which scenario prevails and causes the problem. Indeed, it occurs that the company operates the system with 1 pump open and wrong pipes open. But what are you going to suggest as the best solution?

Suggestion for 1 pump open

Initially you think to suggest scenario 3, but you feel a little bit uncomfortable with it because fluid velocity is limited, it is near to the minimum, so you decide to propose scenario 2, which has sufficient velocity, as the best solution scenario.

Suggestion for 2 pumps open

From the results table you distinct 2 scenarios, 3 and 6, in scenario 3 fluid velocity is higher enough than the minimum while in scenario 6 fluid velocities are close to the minimum. Scenario 6 seems not so stable.  You decide to suggest scenario 3 as the best solution.

Alternatives or additional measures

• Examine scenarios with more pumps, depending on the incoming wastewater pumping requirements
• The above mentioned solutions demand high amounts of energy to deliver wastewater to WWTP
• Suggest a research on the market in order to find more efficient pumping systems using less energy at the same conditions (head and volumetric rate)
• Advice the company to hire highly experienced engineers and technicians in order to maintain and operate correctly the pump station.