Flygt Design Recommendations for Centrifugal Pumps

7/29/2019 Flygt Design Recommendations for Centrifugal Pumps

1/10

Design recommendationsfor pump stations with large centrifugal wastewater pumps


2/10

General considerations for sump designDesign capacity

Sump dimensionsRequired sump volume

Installation tips for submerged pumps

Installation tips for dry installed submersible pumpsSuction pipe design

Installation guidelines for the pump and its piping

ITT Flygt systems engineering

44

68

9

99

10

10

This brochure is intended to help application engineers,designers, planners and users of sewage and stormwatersystems incorporating submerged and submersible,dry installed Flygt pumps.A proper design of the pump sump in such installationsis crucial. Two important design requirements to bemet are; preventing significant quantities of air fromreaching the impeller, and disposal of settled andfloating solids. The Flygt standard pump sump can beused as it is, or with appropriate variations to meet the

requirements of most installations.Pump and sump are parts of an overall system thatalso includes a variety of structures and other elementssuch as ventilation systems and handling equipment.

Operating costs can be reduced with the help ofeffective planning and suitable operation schedules.ITT Flygt personnel and publications are available tooffer guidance in these areas. Transient analysis such asair chamber dimensioning, valve selection, etc. shouldalso be considered in wastewater pump station design.These matters are not addressed in this brochure, butITT Flygt can offer advice on these topics.Please consult with an ITT Flygt engineer to achieveoptimum pump performance, maximum pump life,

and a guarantee that product warranties are met.The design recommendations are only valid for Flygtequipment. ITT Flygt assumes no liability for non-Flygt equipment.

3


3/10

4

General considerations for sump design

Ideally, the flow of water into any pump should be

uniform, steady, and free from swirl and entrained air.Lack of uniformity can cause a pump to operate at alower efficiency rate. Unsteady flow causes the loadon the impeller to fluctuate, which can lead to noise,vibration and bearing problems.Swirl in the pump intake can cause a significantchange in the operating conditions for a pump, andcan produce changes in the flow capacity, powerrequirements and efficiency. It can also result inlocal vortex-type pressure reductions that induce aircores extending into the pump. This, and any otherair ingestion can cause reductions in pump flow andfluctuations of impeller load which result in noiseand vibration with consequent physical damage.Additionally, these fluctuations can impact processloads in other parts of the system.The design of a sump should not only provide properapproach flow to the pumps, it should also preventthe accumulation of sediments and surface scum inthe sump. The following points must be considered: Flow of water from the sump entrance should be

directed toward the pump inlets in such a way thatthe flow reaches the inlets with a minimum of swirland energy loss.

In order to prevent the formation of air-entraining

surface vortices in the sump, the walls must bedesigned and built to avoid stagnation regions inthe flow. A properly placed wall close to the inletcan reduce the tendency toward localised swirland vorticity. The water depth must also be greatenough to suppress surface vortices.

Although excessive turbulence or large eddies shouldbe avoided, some turbulence does help to preventthe formation and growth of vortices.

Sediment, which could be foul, must not accumulatewithin the sump. Stagnant regions, or regions of suchlow velocity where sedimentation might occur, must be

avoided. A sloping floor and fillets or benching oftenhelp to prevent sedimentation. For large variations inflow, part of the sump can be dedicated to low inflowswith a lower floor level and a small pump. Consult ITTFlygt for an optimum sump design.

Surface scum, floating sludge and small debris canaccumulate in any relatively calm region of the watersurface; and this material must be pumped away. Thewater level should be lowered as much as possibleat intervals to increase both velocity and turbulence.However, air should not be drawn into the pump.Please consult with an ITT Flygt engineer in orderto achieve optimum pumping performance. Theoccasional increases in flow velocity will also assist inpreventing the accumulation of sediment on the floor.

Station inflow often approaches the wet well at arelatively high elevation. In such cases, the liquidmay fall a significant distance as it enters the sump.

Such a drop can also occur whenever the pumpshave lowered the liquid level in the sump to thepoint at which all pumps are about to be switchedoff. Therefore, the path between the sump entranceand the pump inlets must be sufficiently long for theair to rise to the surface and escape before reachingthe pumps. The energy of the falling water shouldbe dissipated sufficiently so that excessively high andirregular velocities do not occur within the sump.This can be accomplished with properly designedand correctly positioned baffle walls.

The sump should be as small and as simple asfeasible to minimise construction costs. However, aminimum sump volume may be specified for otherreasons, such as to provide for a minimum retentiontime, or to ensure that only a certain number ofpump starts per hour occur.

Principles to be adopted in the design of any sumpare given in a number of design guides or codes ofpractice for example, both the American HydraulicInstitute and the British Hydromechanics ResearchAssociation have published such guides. Nevertheless,whenever a new design departs significantly fromestablished configurations, model tests of the sump

and its approaches should be considered.

Design capacityA sump designed in accordance with this brochure issmaller than a conventional sump. Consequently, theremay be less buffer volume to accommodate transientvariations of the flow rate. Nor is there extra retentionvolume to store the inflow in excess of the total pumpcapacity (the pipe volumes are usually much largerthan any pump station volume). A proper design of acomplete pump station should therefore consider allcritical aspects of operation.

Of course, the pump capacity must match the extremeinflows to minimise the risk of flooding. Often flowcharacteristics of the feeding sewer system should alsobe analysed.

The control system for the pumps must also provideprotection in the event of a power failure. Precise levelsensors are crucial if the sump volume is minimisedin accordance with the recommendations in thepreceding section.The discharge pipe work should be designed toprevent flooding by the return flow when the pumpsare stopped - also in emergency situations. The effects

of possible pressure surges should be minimised byappropriate design of the control devices.


4/10

Flygt standard sumpA specially designed baffle wall minimisesair entrainment due to falling water.

The flow from the inlet pipe strikes thepartition wall then flows down into theinlet chamber through the slot in thefloor of the baffle. The slot distributesthe flow evenly toward all the pumpinlets. The partition wall is high enoughto ensure that the flow does not surgeover it. Although the flow in the inletchamber is highly turbulent, variousmaterials can collect there. In such cases,side overflow weirs or side gaps maybe used to carry away debris and thusprevent its accumulation. (The top of thepartition wall, or parts of it should bebelow the highest start-level of any of thepumps to allow transport of the floatingmaterial into the pump chamber).Equipping the sump with fillets, baffles,and/or benching is often beneficialdepending on the number of pumps andtheir size. Therefore, please consult withITT Flygt for an optimum sump design.To avoid pre-swirl into the pumpchamber, the inlet pipe must have astraight length of five pipe diameters

upstream from the sump.

The central front high-level entry sumpdesign is referred to as type A1. In thisconfiguration, the flow does not haveto make a horizontal turn, which mightinduce mass rotation in the sump.The exact sump design varies with thenumber of pumps and pump size.

If the piping system and the sump locationdo not allow for a front entry inlet - a side

entry inlet with a baffle wall modified withports can be used. This configuration isreferred to as type A2. In this design, thebaffle wall redirects the incoming flowand distributes the flow evenly toward thepumps through the ports.

A1

A2

5


5/10

Sump dimensions

The sizing diagram is valid for pump stations with up

to four pumps, which may all be duty pumps.Tolerances of 10% on the sump dimensions are

D C F B E A

A1 A1

acceptable provided that the combined effect of thedepartures does not lead to velocities significantlyhigher than those for the standard sump.

Flow per pump refers to the pump duty point when onepump is running alone (in a common pressure pipe).

6


6/10

A2 A2

B2 B2

7


7/10

8

Construction alternativesDistributing the flow evenly to all pumps over the width

of the sump can present problems if the number ofpumps is more than four. In such cases, a double sumpmay be more suitable.

For deep sumps, using a circular outer structure can beadvantageous from a construction viewpoint. Insidesuch a structure, individual pump sump modulessimilar to those used for the compact sump can beaccommodated.

Please contact ITT Flygt for advice of compact sumpdesigns with a central front low-level entry, type B1.

In this arrangement, type B2, with a straight bafflewall, the sewer is below the normal water level in thesump, or an open channel supplies the sump. In theabsence of falling flow in the entrance, no intenseentrainment of air takes place. Consequently, the inletchamber can be greatly simplified because its only taskis to distribute the flow evenly to the pumps.

Sometimes there is a need to locate the riser pipein the centre of the stations or locate the outlet inanother direction, where reversed pump orientationcan be a solution.The picture shows an extract of a reverse orientationwastewater pump station design.When operated in combination with recommendedpump control philosophies optimum pumpingconditions are achieved providing sump floor cleaningfor transportation of solids.

Required sump volumeThe required live volume of the sump, V (m3), i.e. thevolume between the start level and the stop level ofthe pump, depends upon such factors as the cycletime for the pump, T (seconds), the pump capacity, Q

(m3/sec), and the rate of the inflow, q (m3/sec).For one pump and for variable inflow rate, the shortestcycle time occurs if q = Q/2 which gives the minimumrequired volume of the sump:

A1

B2

A2


8/10

To avoid problems with the operation of the levelregulators from floating debris, etc. a stillwell, with itsopening below the lowest water level, may be used.

Sediment can collect in the area behind the dischargeconnections. A steel plate or concrete infill coveringthe pocket will prevent that.

Installation tips for dry installedsubmersible pumps

Suction pipe designThe position of the suction pipe to the dry installed pumpfollows the same hydraulic guidelines as for a submergedpump. The submergence of the inlet must be given moreattention though, as air can be drawn into the pipe moreeasily than a submerged pump, which has a volute actingas a vortex suppressor. Accumulated air in the inlet pipecan impair pump operation, and can cause the system tobecome air bound preventing pumping altogether.For flows over 500 l/s, the inlet pipe should be equippedwith an inlet bellmouth to minimise losses and disturbedflow into the pump.

To achieve a uniform flow to the inlet of the pump, thesuction pipe design should fulfil the following:1. Provide sufficient NPSH

av

2. Minimise friction losses

3. Minimise number of elbows4. Eliminate vapour from the suction pipe5. Ensure correct pipe alignment6. Provide automatic air venting downstream of the

pump before the non return valve

Valve location at the suction pipeTo minimise the risk of cavitation, noise and vibration,the valve and the valve seat should be smooth to avoidflow disturbance (gate valves are preferred), and ifpossible, be located more than five pipe diametersaway from the pump.

The inlet bellmouthTo accelerate the flow smoothly into the inlet pipeand reduce inlet losses, the inlet should be providedwith a bellmouth.

9

The minimum cycle time, (Tminimum) is determined by thenumber of pump starts with regard to the mechanicalstress from the temperature rise in the motor. For Flygtpumps the maximum number of starts per hour is: 605-805 series drive unit 15 starts/hour, corresponding

to cycle time 240 seconds. 905 series drive units 8 starts / hour, corresponding

to cycle time 450 seconds.

T = 3600 / Starts Per Hour

For a pump station with identical pumps, the requiredvolume is smallest if the pumps start in sequence asthe water level rises due to increasing inflow, and stopin sequence as the water level drops due to decreasinginflow. To minimise the required sump volume, thelast pump to start should be the last pump to stop, i.e.cyclic alternation.

Installation tips for submerged pumps

The following are some general installation guidelinesfor submersible pumps:

To avoid check valve cavitation, the valve should notbe located at an elevation greater than 8 meters(approximately 27 feet), above the pump discharge.

To facilitate maintenance and protect the cable, atrough in the floor is recommended for station cableruns (refer to local electrical codes).

The cable support bracket/strain relief sheathingshould be mounted for easy access, i.e. within reachunder the hatch.

When a pipe or a hose is used to protect the cable,it should not cover the cable all the way into thecontrol panel as evaporating explosive gases fromthe wastewater entering the cabinet could be

harmful (refer to local codes). Wastewater gas can cause relay oxidation. It is

therefore beneficial to locate the control panel in aventilated environment.


9/10

10

The optimal velocity at the entrance of the bellmouth is1.7m/s (5.6ft/s), it should be within the following limits.

prevention for dry installed pumps, please seeITT Flygts Installation Recommendations for DryInstalled Pumps.

Pump anchoring and piping support guidelinesThe following guidelines can prevent unwanted vibration:1) All parts of the system should be anchored so the

primary disturbances have frequencies below thelowest natural frequency of the system, includingpump, valves, supports and pipes.

2) If the pump and foundation are to be insulatedfrom the pump station, the following procedure isrecommended:

a) The weight foundations should be at least twotimes the weight of the rotating parts.

b) Use flexible support, e.g. machine feet or a rubbercarpet, between the base and the floor or ground.

c) Use flexible joints for the pipes.d) Anchor the pipes to the floor or to another solid

structure.3) Horizontal and vertical supports should be

provided. Extra supports must be provided at heavycomponents such as valves.

4) For piping systems with bellows to minimise vibration,the pipes should be supported at the bellows to avoidtransference of pressure fluctuations.

5) Vertical pipe velocities must be kept level, whichprevents the settling of solids. The recommended

range is 1.5-2.5 m/sec.

ITT Flygt systems engineering

ITT Flygt provides design assistance on a project basisto owners of pump stations and their consultants.We have broad experience in design and operationof pump stations, and we use unique computerprogrammes developed by ITT Flygt. The scope of ourassistance includes:1. Selection of pumps with consideration of the

variations in the flow capacity and costs involved.2. Optimisation of sump design for given pumps and sites.3. Analysis of complex systems for pump stations

including calculations of hydraulic transients andpump starts.

4. Advice on the need for model tests and arrangementsfor such tests.

5. Advice on achieving the lowest total cost: operation,service and installation costs.

ITT Flygt Systems Engineers are always ready to assistyou in finding the most suitable solution to yourpumping requirements.

Flow

< 1200 l/s

>1200 l/s

Velocity

0.9 - 2.4 m/s3 - 8 ft/s

1.2 - 2.1 m/s4 - 7 ft/s

Installation guidelines for the pump and its pipingAvoiding vibration and noiseRunning a rotary machine will always cause some formof noise and/or vibration. The pump and/or motor unitcan radiate disturbance to the environment that mayresult in vibration and cause noise in other parts of thesystem, pipes etc. With wet-well installed pumps, themedium helps reduce vibration and noise. The designfor dry installed pumps must be more carefully analysedin order to reduce these types of problems.The first rule to follow is that the pump is best operatedin the duty area it is designed for - normally between50% and 125% of best efficiency point (BEP). In thisarea, disturbances such as impeller and volute forces,cavitation etc, are kept to a minimum.The standard pump accessories are designed for usewith a fixed, installation where the main disturbancefrequency will be below the lowest natural frequency,

resulting in low vibration levels.In some cases, a totally fixed design is not enough andthe system or parts of it have to be isolated with rubbermachine feet, a rubber carpet, flexible pipe joints, etc.When evaluating the system, an analysis of the sourceof any disturbance can include: Imbalance in the rotating parts. These have a domina-

ting magnitude at the rotation frequency of the pump. Hydraulic forces that are caused by the pressure

differences in the volute Forces on the impeller and pressure pulses in the

piping system are created as the impeller veins pass

the volute tongue, at a frequency that is the productof the impeller speed and the number of blades.

With this information, it is possible to analyse thesystem in order to minimise the risk for vibration, i.e.the critical pipe length and the minimum distance forthe pipe support to prevent harmonics.Other factors that might create noise are the electricmotor, the internal flow itself (turbulence and swirl cancause pipes and valves to radiate noise) and cavitationwithin the pump or in the pipes/valves.

For more information regarding vibration and noise


10/10

www.flygt.com

ITT Flygt is the worlds leading manufacturer of submersible pumping andmixing solutions.ITT Flygts submersible pumps and agitators are used in wastewater plants,pumpstations, process industries and numerous other applications.

Our experience is utilized by engineers, planners and consultants to ensurereliable and cost-effective utilizations of our systems. ITT Flygt has service andsales facilities in more than 130 countries.

C-Design.0

2.0

6.

Eng.

2M.0

4.0

3I

TTFLYGTAB

PrintedinSwedenKT230374

891219

Flygt Design Recommendations for Centrifugal Pumps

Documents

Transcript of Flygt Design Recommendations for Centrifugal Pumps