PUMP MINIFLOW - Factors affecting it & protection strategies

REFRESHER

Factors affecting pump minimum flow
Experience engineers aware that Centrifugal pump will experience to cavitations when flow rate reach it minimum allowable flow. What are factors affecting pump minimum flow other than internal circulation?

There are five known factors will affect the minimum flow rate of a pump.
a) Fluid temperature rise
b) Minimum stable flow
c) Internal recirculation
d) Thrust capacity

Fluid temperature rise

As fluid enters pump suction chamber, pump impeller will spin the fluid and energy from shaft / impeller is transferred into fluid. However, there are mechanical losses within the pump in the energy transferring process. The mechanical losses will be transformed into acoustic energy (e.g. noise) and thermal energy (e.g. fluid temperature rise). Acoustic energy may transfer to fluid, pump casing and piping and it is further emitted to atmosphere as noise. Similarly thermal energy will also transfer to fluid, pump casing and piping. However, pumping process is a rather “fast” action and in general there is “insufficient time” for heat transfer from fluid to pump casing and piping. Thus, majority of the thermal energy will stay in the fluid and eventually cause fluid temperature rise (and/or liquid flashing). Reduce fluid passing pump results less heat “carrier” and higher temperature rises and high potential of liquid flashing. Thus a minimum flow can be established from fluid temperature rise.

A process engineer may needs to establish minimum flow of a pump from temperature rise perspective. The principle is rather simple where

Energy loss to fluid = fluid thermal heat gain

Above has considered acoustic energy is zero.
Fluid temperature rise,

Where

h = pump head
g = gravity acceleration
e = pump shaft efficiency
Q = pump flow
Cp = fluid specific heat

Minimum system stable flow

Sometime a pumping system may shows two stable flows at certain pump head. As a result, it ”hunts“ or ”shuttles“ between these two flows and potentially damage the pump and other equipment within the pumping system. For example, gas trapped in the discharge line pocket. Trapped gas will reduce liquid flow path and increase line pressure drop. Pump head increase push trapped gas towards downward piping. As trapped gas move into the downward piping, liquid flow path increases and reduce the pump head. Trapped gas will form smaller bubble due to fluid turbulence and it will rise in the downward piping and finally back to the high pocket again. Similar cycle occurs again and pump oscillate in two flows. A process engineer shall always piping system to avoid pump operate in the oscillation region.

Internal recirculation

As flow rate decreases in the pump chamber, flow reversal will occur at the pump suction and discharge vanes. Recirculation vortex will form at both ends and potentially damage pump. Thermal heat gain within vortex will further increase pump fluid temperature and potential flashing occurs.

Axial thrust load

Axial thrust in a pump increases rapidly as flows are reduced and head increased. A minimum flow needs to be maintained so the thrust developed by the pump does not impair bearing life.

Minimum flow protection strategies
There are number ways to implement minimum flow protection strategy for centrifugal pump. Typically they are:

(i) A restriction orifice on pump discharge recycle line
(ii) A flow meter on pump discharge with control valve on recycle line
(iii) Use Automatic Recirculation Valves (ARC) valve
(iv) Flow-Delta P and flow meter on pump discharge with control valve on recycle line

Fig. 1 restriction orifice on pump discharge recycle line


Fig. 2 Flow meter on pump discharge with control valve on recycle line



Fig. 3 Automatic Recirculation Valves (ARC) valve




Fig. 4 Flow-Delta P and flow meter on pump discharge with control valve on recycle

COMPARISON
The following list out the advantages and disadvantages for above four options

Restriction orifices
- Simple installation
- Maintenance free
- Large pump
- Waste energy all time
- Limit maximum pump output.

Orifice plate in the discharge line
- No continuous recycle thus energy saving
- consumes energy and also slightly reduces pump capacity.
- Using orifice plate to measure flow will results high inaccuracy e.g. minimum flow is 40% of maximum flow, 7% of the set point may be expected.

ARC valve
- Simple & effective
- Lack of flexibility.
- Unstable operation
- expensive

Sketches below show outlook and operation of an ARC valve

Flow-Delta P
- Similar to orifice plate option
- Good option for load sharing of pumps in parallel installation

Restriction orifice option is the most common option adopted in many applications with LOW capacity system and a process engineer is advisable to consider this option. However, for HIGH system and appreciable energy losses is possible, a process engineer is advised to consider flow orifice with control valve option. ARC valve may be a good option to consider if process engineer is well aware of the fluid characteristics and familiar with the operation & dynamic of ARC valve. For very high capacity system and load sharing may be expected, process engineer may consider Flow-Delta P option.

Reference :

S, Mirsky, << Pump Control Strategies Benefits from Compressor Know-How >>, Hydrocarbon Processing, Feb 2005

Walter Driedger, <<CONTROLLING CENTRIFUGAL PUMPS>>, May 2000

TYCO, <<YARWAY 9100 Series ARC Valve For Centrifugal Pump Protection>>,



JoeWong

What are the conditions causing pump cavitation ?

Five main conditions may result pump cavitation and potentially damage pump impeller, chamber and bearing. Understanding of factors causing and affecting these conditions is rather important as it assist in troubleshooting pump cavitation problem. They are :

• Vaporization
• Gas and/or non-condensable fluid entrainment
• Internal recirculation
• Flow turbulence
• The Vane Passing Syndrome

Vaporization .
Pump fluid vaporizes when operating pressure drop below its vapor pressure at specific flowing temperature. A process engineer must always keep this in mind and always to ensure pump fluid will be vaporized at minimum operating pressure along fluid path from suction vessel/tank to pump chamber at worst flowing temperature. Net Positive Suction Head (NPSH) is a parameter used to measure possibility of cavitation. A process engineer must always ensure NPSH available must always higher than NPSHr.

NPSHa > NPSHr

Obviously in order to maintain above requirement, a process engineer may aim to INCREASE NPSHa and / or DECREASE NPSHr.

Increase NPSHa
Now we shall further look into the possibility of increasing NPSHa :

Net Positive Suction Head (NPSH) available define as follow :

Process engineer should target to increase Suction static head & Operating head (pressure) and decrease fluid vapor pressure @ flowing temperature and frictional head.

As discussed earlier, there are few ways to increase NPSHa :

a) Increase suction line size to reduce friction head loss. Generally flow velocity is less than 1 m/s

b) Rearrange and /or redesign suction pipe work to minimise bends, valves and fittings

c) Raise suction vessel

d) Increase & maintain pressure in suction vessel

e) Reduce fluid vapor pressure i.e. subcool fluid In additional, process may pay consider the following :

f) Install a booster pump (if necessary)

g) Do not installed over-capacity pump (after suction line size is fixed)

h) Injecting a cooler fluid at the suction vessel (if practical) i) Insulate piping to avoid solar heating

j) Avoid recirculation line directly feed to pump suction line as pumping may raise fluid temperature

k) Use 45° elbows instead of 90° elbows to reduce friction

l) Do not select overly fine screens or intake filters and choose cleanable screen m) Avoid pocket at pump suction line

n) Ensure correct gasket installed at pump suction and tie flange to minimize air ingress

o) Use eccentric type reducer instead of concentric type

Sometime pump cavitation will only occur after the pump is operated for some period. Process engineer may look into the following factors :

- pipe liner has collapsed or solid / corroded material built-up and blocking suction strainer

- Tank vents blocked causing pressure dropped. Vent can be blocked frozen ice in cold weather, bird & insert, etc

- A bigger pump has been installed on existing system cause high line loss

- Install globe valve instead of gate valve at pump suction

- gasket protruding into the pipe

- Increased pump speed especially variable speed drive pump

Decrease NPSHr
AS discussed earlier, NPSHr is very much subject to design and construction of a pump and upto manufacturer. Nevertheless, Process engineer may advise to consider and include in the process design with the follow choices :

- Use a double suction pump where NPSHr can be reduced by almost 25%
- Use low speed pump
- Use pump with large impeller eye opening
- Install Inducer (inducers can cut NPSHr by almost 50%)
- Installer smaller pump by adopting 3 x 50% instead of 2 x 100%. Normally smaller pump may required lower NPSHr

Process engineer is advised to take some margin on the NPSH. The margin of 1 m is recommended for vaporization case.

Gas entrainment
Gas entrainment from suction vessel into pump fluid has an adverse effect on the pump impeller. Entrained gas will form bubbles in the flowing fluid and the bubbles will collapse as they pass from the eye of the pump to the higher pressure side of the impeller. Collapse of entrained gas as compare to bubble formed due to vaporization, it has lower impact to the pump impeller. The main effect is entrained gas is capacity loss.

There are number of ways may result gas entrained into pumping fluid :

• Through the packing stuffing box. This occurs in any packed pump that lifts liquid, pumps from a condenser, evaporator, or any piece of equipment that runs in vacuum
• Valves located above the water line
• Through leaking flanges
• Pulling air through a vortexing fluid
• If a bypass line has been installed too close to the suction, it will increase the temperature of the incoming fluid
• Any time the suction inlet pipe looses fluid. This can occur when the level gets too low, or there is a false reading on the gauge because the float is stuck on a corroded rod.
  

Internal recirculation
Internal recirculation occurs at leading edge of the impeller, close to the outside diameter, working its way back to the middle of the vane. Similar condition occurs at pump suction eye as well. Fluid recirculates will results its velocity increase and subsequent pressure drop until it vaporizes. Bubbles may collapse quickly at the surrounding higher pressure region within pump impeller.

Flow turbulence
Flow turbulence is always occurred of the pump suction with impeller impacting the incoming fluid. Change of incoming fluid will change velocity and it operating pressure and results short term cavitation.

The Vane Passing Syndrome
Vane passing syndrome and causing cavitation occurred when the OD of the impeller passes too close to the pump cutwater. The velocity of the liquid increases as it flows through this small passage, high velocity lowering the fluid pressure and causing local vaporization. The bubbles then collapse at the higher pressure region just beyond the cutwater. This phenomenon occurred at the center of the impeller vane.

JoeWong