Your questions answered: Vertical Turbine Sump Design

Jimmy Scroggins, technical training expert at Grundfos Pumps Corp. in Lubbock, Texas, tackled unanswered questions from the Oct. 21, 2015, webcast "Vertical Turbine Sump Design."

Q: What is your experience in vibration analysis testing for vertical turbine pumps when the motor is mounted on the side of the vertical pump assembly?

Jimmy Scroggins: In my more than 20 years of experience, I have never seen a side-mounted motor on a vertical turbine pump. I have seen horizontal applications where both motor and pump are horizontal, and in that, reed-frequency calculations were fairly straightforward. The best bet would be to use a finite element analysis (FEA) to determine natural resonant frequencies of the driver/pump combination compared to job speeds. I would make sure to have good 3D models (IGS or STP formats) of the pump, foundation, motor, and attached piping with supports to give to the engineer or firm conducting the FEA (saves time and money).

Q: In NPSH available calculations, what are typical values for Ha, Hst, and Hfs that ensures optimum NPSH required?

Scroggins: Those values vary widely based on a number of factors:

Q: What steps would you recommend when you are retrofitting a new pump into an existing installation where the installation has been operating for many years and the same pumps are no longer available?

Scroggins: This is a tricky situation. Hopefully, you can get a bill of materials—especially a general-arrangement drawing—and anything else that would be provided in a typical submittal package. Please note that many pump manufacturers provide leaded red bronze impellers and bearings in the not-too-distant-past, but may not offer those same materials anymore. If you don't have the submittal data, you might want to treat this as a new application and request installation data, such as sump dimensions, electrical data, pumping information (to get correct pump materials), and any special design requirements (parallel operation, VFD, secondary and tertiary design points, shutoff head, and runout flow limits, etc). Good luck.

Q: Does Grundfos have computational fluid dynamics (CFD) analysis software to aid engineers in analyzing intake structure design?

Scroggins: Yes we do. Contact your salesman or distributor.

Q: In which cases must we use a can pump?

Scroggins: Can pumps are fantastic for low NPSHA and high pressure applications. You can (pun intended) increase NPSHA by simply lengthening the can-and-column assembly to squeeze more Hst out of the application when confronted with low NPSH applications. Also, the can serves as a secondary pressure device, sparing special material and geometry bowls in high pressure applications (the pressure inside the can helps offset the pressure produced by the bowl assembly).

Q: Why do we always use a vortex suppressor as a strainer on can pumps?

Scroggins: The amount of space between the bottom of the can and the bell lip is very limited in a can type application. A regular basket strainer is usually very long and designed to fit in an open sump. Also, ANSI/HI and AWWA dictate the size of the basket strainer, making them cumbersome in a can-type application. ANSI/HI allows for a very short basket strainer to be used in can applications—what you correctly called a vortex suppressor. Can applications, without straightening vanes, have a tendency to allow the pumpage to swirl around during operation and will feed the suction device in one area, which creates problems hydraulically for the pump with subsurface vortices and pre-swirl. The vortex suppressor breaks up the flow as it enters the pump suction, reducing or eliminating vortices and swirl (less noise, less wasted motion, better flow, better efficiency, less vibration, etc.).

Q: Which is the preferred analysis of pump sump: physical scale modeling or CFD?

Scroggins: If money is no object and there is plenty of time for a physical scale model, it is the best. Otherwise, CFD analysis works very well with clear liquids like water with no suspended solids. A CFD is not cheap (usually $5 to $15,000), but is much less expensive than a scale model. I hope that helps.

Q: What are the most common problems with vertical turbine sumps from an operational standpoint?

Scroggins: There are a few disadvantages with vertical turbines (it pains me to say that). But a VTP requires high headroom, and they can be challenging to install or remove. In addition, meeting the minimum fluid level for priming, NPSH, and submergence can sometimes be interesting. Also, because they are usually long and thin, vertical pumps are susceptible to high vibration. But in my opinion, the advantages of vertical turbines far outweigh their disadvantages.

Q: For a vertical turbine pump in a water well, can we consider the well casing the same as a sump?

Scroggins: Not exactly. For well pumps, flow usually comes from the bottom up. Sump pumps draw in water in horizontal planes. This is a very subtle difference, but it's huge hydraulically when dealing with vortices, uniform flow, stagnant zones, etc.

Q: Should the pump intake be no closer than 5 pump diameters from the well screen?

Scroggins: Actually, ANSI/HI 9.8-2012: Rotodynamic Pumps for Pump Intake Design sets a screen at four suction bell diameters from the suction bell centerline for clear liquid rectangular intakes.

Q: If you suspect that cavitation is occurring, are there any signs that can be observed when near the motor?

Scroggins: That is a good question. I will check with one of our field service representatives and provide a follow up. Typically, cavitation increases vibration and load fluctuations, which would have to manifest themselves in the driver. This gets back to the same symptom for many different diseases.

Q: Can you use variable speed motors on vertical turbine pumps?

Scroggins: Yes, very much so on short sets, such as in a vertical sump. But on deep sets or wherever there is a high lift, the pump must overcome, I would not recommend using a variable frequency drive (VFD).

Q: What are the advantages of a vertical turbine pump vs. submersible sump or other type of pump?

Scroggins: The advantages for a vertical turbine pump are: a smaller footprint, no priming issues, low NPSHr impellers are available, submergence and NPSHA issues are eliminated by simply lengthening the pump, pressure flexibility by adding or removing stages, reduced noise, and vertical turbines readily lend themselves to customization to meet existing system needs. The disadvantages or what I like to call opportunities are few. But a vertical turbine requires unusually high headroom: they can be a bit challenging during installation and removal; they must meet a minimum fluid level for priming, NPSH, and submergence; and they are susceptible to high vibration.

Q: How do you take into account pressure drop caused by flow straighteners? Is there data available to include that pressure drop in the NPSHA calculation?

Scroggins: At this point, you would need to run a CFD analysis to determine the pressure drop. And I do not know that any pump company has addressed this issue, nor have ANSI/HI or AWWA. But you do pose an interesting question.

Q: In applications where NPSHA is very close to NPSHr, can flow straighteners be counterproductive?

Scroggins: It is possible, but I would think that a CFD analysis might show a net benefit tilting slightly in favor of flow straighteners. It's very good point, though.

Comment: Other considerations when adding stages include the additional horsepower required and the shaft strength required for the increased horsepower and down thrust.

Scroggins: This is true. And when designing for future state, those parameters should absolutely be considered.

Q: We are involved in design of large power plants in which circulating water pumps each of 130,000 gpm or larger are used. Will the same approach of sizing sumps apply to these installations?

Scroggins: Yes. In fact, ANSI/HI 9.8 offers guidelines up to 300,000 gpm. But projects of that scale—unless duplicating another cooling tower application—would be a great candidate for CFD analysis, and possibly a scale model. I would suggest at least a CFD analysis, and if the results are marginal, then either use a scale model or a design change. Good luck.

Q: What are the codes and standards useful for design of sumps?

Scroggins: There are very few standards and codes. Even our Sump Design Group in Denmark uses ANSI/HI 9.8. But others I recommend include:

I consider those to be a very good start. A good CFD software program might have additional information, such as an ANSYS fluid-modeling book.

Q: Could you offer any considerations for a trench style self-cleaning wet well?

Scroggins: There is a cleaning procedure for trench-type wet wells in ANSI/HI 9.8 (Section 9.8.3.2.3.5, to be specific) for solids-bearing fluid applications. I am unable to offer any useful insights to this procedure because I have not worked with this type of sump using solids-bearing fluids. All I could offer is to parrot what is mentioned in the ANSI/HI standard.

Q: Can you briefly discuss clearance requirements for canned turbine pumps to prevent cavitation?

Scroggins: To be brief would be impossible for this particular subject, so please forgive me. I will interpret clearance requirements as dimensioning for can-type pumps. For flows greater than 5,000 gpm a CFD analysis would be recommended and for flows greater than 10,000 gpm, a physical scale model is recommended. For flows of less than 5,000 gpm, there are few things to consider. First, a few terms and definitions:

A plain can (tee head or suction nozzle located in discharge head):

Note that in a can pump, can suction losses are NOT negligible and must be subtracted from NPSHA (obtain from the pump manufacturer).

Suction in can (ANSI-mounted head or suction nozzle located in can):

Notes:

*The straightening vane inside diameter should be as close as possible to largest outside diameter within the can. I suggest using SVid = 1.08 × Dx, or at the very most 1 in. between the vane and the suction bell lip, the bowls, or the column flange.

**Because of the limited volume in a partially filled can, surging of the fluid level could pose a problem. Intake piping should be large enough to limit drawdown below the minimum fluid level for no longer than 3 sec at startup.

Q: Vertical turbine pumps used in fire protection systems are typically fit into small sumps built into prefab tanks. Based on 2,500 gpm pumps, how big should such a sump be?

Scroggins: I do not have enough information to correctly answer this question. It depends on the minimum duration time × 150% design flow (3,750 gpm) rounded up to the next size tank. I would check NFPA 22: Standard for Water Tanks for Private Fire Protection or the authority having jurisdiction.

Q: Is the first stage impeller in a multistage pump the top or bottom impeller?

Scroggins: It is always located at the bottom. Note that NPSH is a function of the first stage impeller only.

Q: Is submergence required to avoid air-entry to prevent vortex in the sump measured above impeller inlet eye or above the level of bottom of intake bell?

Scroggins: That is a little confusing, I agree. On the first slide for submergence, I hope you noticed that the dimension line started at the bell lip. That's because vortex suppression submergence (avoidance of air entry) is measured from the suction bell inlet. The suction bell inlet is where the vortex action begins. The dimension line for NPSH did not fall to the bell lip, but just above it. That's because NPSH submergence is measured from the leading edges in the eye of the first stage impeller (where the dimension line was placed). NPSH required is based on the velocity of pumpage through the first stage impeller eye. And in the NPSHA calculation, the static head (HST) is the NPSH submergence measured from the first stage impeller eye to the fluid level.

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