reverse flow

7/31/2019 Reverse Flow

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Theory of Reverse flow

The flow reversal is an known fact in process plant, when there is sudden power cut to the pump driver clubbed with mal-functioning of one way check valve at the discharge line. The pumps having higher specific speed are prone to experience

higher reverse speed in this process, in case of sudden power failure and having faulty check valve.

Subsequent to power cut, the only drive left to rotate the pump in forward direction is, inertial force of the rotating elements.

Since the energy is relatively small to maintain the flow and also gradually diminishing, as the rotating elements have toovercome frictional as well as electro-magnetic forces due to residual magnetism of the motor field. Hence the pump speed

reduces very rapidly causing rapid reduction of flow, which creates rapid flow changes followed by water hammer waves ofincreasing subnormal pressure at discharge line. Soon the speed of the pumps arrives at such a point, when no flow could be

pushed against the existing head.

At this point the flow of fluid tends to reverse its direction and pass through the pump chamber from discharge to suction, ifthe check valve is faulty. This happens while the pump is about to terminate its rotation in forward direction.

This reverse flow compels pump to drop the forward rotation and passed over to rotation in reverse direction crossing zerospeed zone. Now the pump starts functioning like a rection type hydraulic turbine with no load and gradually reaches to

runaway speed.

Runaway speed is the maximum possible speed a turbine could reach in case of sudden load drop and failure of governing

system. That means it is maximum speed of turbine in no load condition under maximum available head and control valvefull open. Normally the runaway speed for a reaction turbine is 2- 2.2 times higher than rated operating speed.

A radial flow centrifugal pump is a reverse rection turbine; hence while reverse flow takes place the above-mentionedphenomena

Also applicable to this.

Theory of work done for centrifugal pump with radial flow vane.


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The centrifugal pump is a device to convert the mechanical energy to hydraulic energy to enable liquid mass to travel fromlower energy state to higher energy state.

This action is based on mass flow and impulse momentum theory, which is fundamental of centrifugal force.

The impulse exerted on any body or mass is equal to the resulting change in momentum of the body.

The derivation based on Newtons second law of motion leads to the equation

Force = Mass X Acceleration = Change in momentum in the direction of motion over a period of time

Similarly In the case of rotational motion,

Torque = Moment of force = Rate of Change of angular momentum in the direction of rotation over a time periodWork done by an impeller per sec. = Torque x angular velocity

Angular momentum principle

dFx

dm

X

dFy

Y

Let a mass of liquid dm is rotating about Z-axis. Vx and Vy are the its velocity component in x and y direction.

Z

Ax=acceleration component= dVx/dt and Ay=dVy/dt . Applying Newtons 2nd

law of motion to the fluid mass

dFx =dVx/dt .dm and dFy=dVy/dt .dm


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dFx and dFy are the external forces acting on the liquid mass causing acceleration. The moment of external force about zaxis(counter clockwise being positive) or the torque about z axis

dTz =(xdFy-ydFx) =( xdVy/dt ydVx /dt ).dm.

By rules of differentiation

d/dt(xVy-yVx) =dx/dtVy-dy/dtVx +xdVy/dt ydVx/dt = VxVy-VyVx +x dVy /dt ydVx /dt

now VxVy-VyVx =0 and dm is constant ,hence dTz = d/dt[(xVy yVx) dm]

The quantities (dmVy)x and (dmVx)y are moments of momentum or angular momentum. Therefore the rate of change of

angular momentum about any axis is equal to the torque about the same axis.Applying the same principle in the case of pump the effective torque about the axis of rotation and work done on the liquid

can be calculated as mentioned below.

Ui

Vri

Vfi

Uo

VoVwo

Vro

Ri Ro

Axis of rotation

Direction of

rotation


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ViAbsolute Velocity of flow at inlet

Vri-Relative velocity at inlet

Vwi-Velocity of whirl (horizontal component at inlet in the direction tip tangential velocity)

Vfi- velocity of flow component at inlet

Ui- vane inlet tip tangential velocity in the direction of rotation

VoAbsolute Velocity of flow at outlet

Vro-Relative velocity at outlet

Vwo-Velocity of whirl component at outlet (horizontal component in direction of tip tangential velocity)

Vfo- velocity of flow component at outlet

Uo- vane outlet tip tangential component velocity in the direction of rotationRi- Radius of curvature inlet profile

Ro-Radius of curvature outlet Profile

wa- angular velocity of impeller

For given mass flow W/g per sec. the momentum of liquid about axis of rotation at inlet and out let vane are W/g x Vwo

and W/gVwi. The moment of momentum at inlet and outlet tip are W/g x VwoRo and W/gVwiRi

Hence torque developed by impeller vane about the axis of rotation = Rate of change of moment of momentum about the

axis of rotation= W/g x (Vwo.Ro-Vwi.Ri)Work done= Torque x angular velocity= W/g x (Vwo.Ro-Vwi.Ri) x wa

Again Ui=wa.Ri & Uo =wa.Ro

Hence work done by impeller= W/g x (Vwo.Uo-Vwi.Ui)

In case radial entry at inlet the inlet angle is 90 degree.


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Now Vwi= Vi cos 90= ZeroHence the work done by impeller = W/g x Vwo.Uo In case of reverse flow while the pump functions as inward flowreaction turbine the fluid enters at outer tip at an angle equal to vane angle at outlet and exits from inner tip radially causing

the horizontal component of exit velocity equal to zero. In such case the work done on the runner for given flow= W/g x

Vwo.Uo = wQVwo.Uo

Horse power=wQVwo.Uo/ 75 -----------------(1)Q= volume flow and w=specific weight

Where Vwo- Velocity of whirl component at outer tip

Uo- Runner outer tip velocity

The power transmitted through pipeline

Pipes carrying liquid under pressure from one point to another may be utilized to hydraulic power. The head available at the

outlet of pipe = H-hf

Where H--- Total head for power transmissionhffirctional loss

If Q volume of flow, D diameter of pipe, V be the velocity of flow and L is total length of travel through the pipe line ,then

hf= fLV2 / 2gD and Q=D2 / 4 x V

The power( or energy per sec.) available at the outlet of the pipe

=wQ. (H-hf) = w(D2 / 4 x V )(H-FLV2 / 2gD)

Now neglecting other losses the maximum power transmitted through pipeline may be obtained by differentiating aboveequation in respect to V and equating to zero.

d. Power/dV= w (D2 / 4 x (H-3FLV2 / 2gD)=0

or (H-3FLV2 / 2gD)=0

or H=3hf


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Hence head available at outlet of pipe will be 2/3 HHorse Power = 2wQH /3 x75 ---------(2)

The efficiency of hydraulic turbine of reaction type is between 85% to 90%

From equation (1) &(2)

0.9 = wQVwo.Uo/ 75

2wQH /3 x75

From above equation the value of Uo could be obtainedUo=DoN /60

N=Uo / 3.14 x 60

N= reverse rotational speed

From above equation

Uo =1.8 x H / 3 Vwo or x Do x N /60 = 0.6 x H

Or N = 3.6 x H / 3.14 x Do x VwoValue of Vwo may be calculated by measuring the vane angle at outer tip ()And absolute velocity V= Q/ Do2 /4

Vwo =Vcos

= = outlet vane angle of pump = Runner inlet vane angle when acting as hydraulic turbine


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H1

Z

H2Velocity=V

Pr.head= zero

Potential head= zero

Velocity head= zero

Pressure head=differential head

Potential static head= Z

Neglecting the losses due to whirl and piping losses other than friction and free flow the absolute velocity at inlet to the volute chamber duringreverse flow can be obtained by applying Bernoullis at the points H1 and H2. When the reverse flow velocity attends maximum value.

H+Z-hf = V2/2g OR H-H/3+Z V2/2g or V2=2g(2/3H +Z) or V= 2g(2/3H+Z)

H the differential head-- In case of inward flow radial reaction turbine will be not more than rated pumping head while operating as pump.

Hence knowing V and vane angle at outer tip Vwo can be calculated.


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Higher the specific speed higher is the reverse speed. Hence axial flow pumps reach to very high speed in

comparison to radial flow pumps.

Effects of reverse rotationThe reverse rotation of pump causes sever damages to pump and motor. Few of them are listed below.

Pump

Bearing failure

Mechanical seal failure

Damage to pump shaft/Impeller/wearing

Damage to bearing housing/stuffing box

Motor

Bearing failure

Damage to Armature

Damage to end ringDamage to stator Winding

When reverse flow passes through the impeller accelerating its rotation in opposite direction, it creates substantial

amount of hydraulic imbalance inside the casing across the impeller. The unbalance hydraulic forces give rise to radialand axial load of variable magnitude and direction acting on rotating parts and static parts as well. The thrust bearing


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cause circulation of higher leakage current and winding temperature rise. Higher the speed higher will be induced emf.High-induced voltage may cause the ionization of air gap between winding terminals, damage of insulation and leading

to a disastrous situation.


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Failure of Fractionator bottom Pump &Motor of Delayed cocker Unit


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Equipment details

PumpMake- Sulzer weise Germany

Model-GSG 100-287/ 7 Stage

ServiceReduced Crude Oil

Suction pressure-2.2 Bar / 4.5 Bar

Discharge Pressure- 54.45 Bar / 57.20 Bar

Pumping Temperature370 degree Celsius

Sp. Gravity-737 kg/m3 at PT

Capacity176 m3 / hr.

NPSH r4.7 meter

Lubrication- forced lubrication

Bearing Radial-- journal (Babbitt lined)Bearing Thrust- Tilting Pad

Shaft seal- Mechanical seal

CO

L

U

M

N

Flushing Plan 32

Driver- Motor

Make- Simens Germany

Operating voltage- 6.6 kV

Power- 450 kW

Type- 1LA 1404-2HE 70-Z


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General flow diagram of the system

Bottom

Pump

FurnaceFour Pass (30

tubes per Pass)

BFW injection


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IncidentThere was a power failure at LT substation. All LT motor stopped. Though the subject equipment motor is of HT supply,

but operator stopped it from panel switch as there was no supply of external seal oil. While closing the discharge valve after

stopping, the pump was observed to be rotating with high speed and high noise was also audible from it. The direction of

rotation could not be ascertained due to lack of illumination. This state was noticed when discharge valve was closed only

25%. Operator also reported deflection in local ammeter pointer during this period. The puzzled operator left for control

room and this condition had prevailed till a second operator came and closed the valve. At the end of closing operationheavy smoke and oil came out from non-drive end bearing. Smoke also came out from motor junction box with burning

smell. The pump came to stop after full closer of discharge valve.

It was also reported that following the power failure, which caused tripping of furnace, boiler feed water of 30 kg/cm2 was

injected to discharge line before going to furnace to avoid coking in the furnace coil.


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The time lag between power off to this pump and full closer of discharge valve is 10 minutes.

Observations

Motor condition after dismantle

The rotor armature got bulged at drive at drive end side.

The rubbing marks on armature curvature are of clockwise impression while looked from non-drive end side.( normaldirection of rotation is counter clockwise looking from NDE)

Uniform groove noticed on stator yoke at drive end side.

Armature end ring at drive end got melted

Fins of cooling fan found fully damaged and deformed by rubbing action

Stator winding found damaged by mechanical rubbing

DE bearing appeared visually o.k. but NDE bearing got completely damaged and dislodged from position on shaft.Some wear of damaged fins had elongation marks in clockwise direction while looked from NDE.

Rotor cooling slots found partially choked with aluminum dust

No impression of burning found on rotor/stator

The motor terminals found completely deformed due to over heating and terminal lugs got melted.

DE side of rotor got bulged along the circumpherence with bluish mark .

Pump condition after dismantle

Both pump bearings found completely damaged.

Mechanical seal one side found over compressed and other side got opened.

The impellers got touched with diffusers indication shifting of rotor.

The non-return valve at discharge line found stuck in open position.


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AnalysisIt is evident from the observations that severe damages occurred to the motor mainly because of generation of hightemperature,

Shifting of rotor due to bearing damage and mechanical rubbing of different parts of armature and stator.

If the facts of observations like high speed rotation and noise after power off, the extrusion of damages on armature and

fins in direction opposite to normal direction of rotation, stuck NRV in open position and smoke from terminal box of motor

are considered and correlated, it instantly directs the analysis towards the possibility of reverse rotation of pump and motor.

If the sequence of occurrences are backtracked, the following factual event flow diagram is obtained.

Reverse rotation of pump under

discharge head, as NRV&

discharge valve was open

Simultaneous injection of

30kg/cm2 BFW to discharge

line for furnace coil.

BFW back up in the pump maintainingthe reverse rotation and sustained

period at runaway speed, which is

much higher than normal RPM of

pump.

Induced e.m.f on stator due to high rpm

rotation of armature with residual

magnetic flux, Generating high voltage

across the terminals.


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Temperature rise in stator winding due

to impedance & its function as open

circuit generator and subsequentshortening of stator terminals due to

high voltage ionization.

Induced voltage voltage drop due to

Stator impedance

Phase angle

Terminal

voltage

Stator current


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The deflection in local ammeter observed by operator also confirms the fact of momentary flow of current due to emf

induced. This is possible when the current transformer installed on down streamside of the circuit breaker (Motor side).

Circuit Breaker

CT &Ammeter

Motor

6.6 KV

Bus bar


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Metallic failureThe high temperature generation along with vibration due to resonance are the possible cause for metallic deformation at

both DE side and NDE side. It may be noted, the normal operating RPM of rotor remains between Ist and 2nd critical speed

for rotor with flexible shaft. Hence during speed up on reverse rotation the most possibly it had undergone the phase of

critical speed resonance experiencing high vibration level. Now it is not known that how long it had been in the 2nd criticalsped range as this depends on torque applied for speeding up and the damping effects due to various frictional and electro-

magnetic forces.

Aluminium powder formationFormation of aluminum powder was due to rubbing of fan fins with irregular surface of stator windings during the course of

rotation, as copper made winding strips are of higher hardness than that of aluminum. Subsequently it got deposited insidecooling slots of rotor.

Groove in stator yoke at DE side


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Basic cause of this is the bulging of rotor surface due to high temperature and rubbing of localized bulged portion of rotor

with stator yoke. This is further confirmed by the bluish heat marks on the rotor periphery.

Considering all above facts it could be undoubtedly concluded that the, malfunction of Non-return valve and stoppage

of pump without closing the discharge valve first causing reverse rotation of pump is the Root cause of failure.

Recommendations1) Centrifugal pump having radial vanes of higher capacity and head to be stopped by after closing the discharge valve.

This must be included in operation checklist and emergency handling procedure.

2) Periodic preventive maintenance of NRV and discharge valve to be carried out to ensure proper function.

3) Installation of remote operated valves on discharge lines for pumps of higher capacity and located far from control

room.

Brief Introduction of Author

Name: Sourav Kumar Chatterjee


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Senior Manager Reliability HPCL MR

Educational backgroundChartered Mechanical engineer (Associate Member, Institute of Engineers India) & Qualified Boiler proficiency engineer (BOE) with

Diploma in Mechanical & Electrical Engineering and B.Sc from The Calcutta University

Experience BackgroundPossess more than twenty-four years experience in thermal power plant operation and maintenance including boilers, steam and gas turbines and

generators and maintenance of refinery rotary equipment. He specializes in failure analysis and equipment reliability.

Academic activitiesPresented many papers in, National conferences organized by PGIEEM Mumbai, Chemical Industry Digest Mumbai, HIMER Process Plant

Maintenance conferences Chennai." Alliance India Asset management conference 2005 Mumbai, Plant operation& Maintenance Club organized

Workshop on condition monitoring of Rotating Equipment 2005 at Varadhara etc. He has also presented papers in International conferences of

NPRA 2001 Maintenance conference USA, Hydrocarbon Asia Bottom line improvement conference 2003 Singapore, Marcus Evans Plant Reliability

& Maintenance Conference 2005 at Kuala Lumpur, Malaysia & 2006 at Mumbai, Refinery Technology Meet 2004 & 2005, Oil &Gas IQPC

rotating equipment conferences 2006 at Kuala Lumpur and has published nearly papers in hydrocarbon Asia Singapore, Chemical processing

USA, Chemical Industry Digest India Pump Magazine USA, Hydrocarbon Processing USA maintenance world USA etc.

Nominated Activity committee Member of Rotary & Reliability Committee -Center for High Technology A registered society under ministry ofpetroleum Govt. of India

Associate Member of Institute of Engineers India

Advisory Committee member of Plant Operation and maintenance Club

Nominated committee Member for reviewing standards for Centrifugal pumps/ compressors under Bureau of Indian Standard (BIS)

Nominated functional committee member under Oil India Safety Directorate for review/ formulation of safety Standards on maintenance practices.Pumps, compressors, Mechanical seal etc.

Present Organization- Hindustan Petroleum Corporation Limited India

Title: Senior Manager Reliability

Address:- Mumbai Refinery , B. D Patil Marg Mahul Chamber Mumbai -400074 India


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Phone: 91-22-25076270

E-mail- [email protected]

reverse flow

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