selection and operational considerations
TRANSCRIPT
Selection and Operational Considerations
Harold Streicher,
Vice President Sales
Hansen Technologies, Inc.
Introduction to liquid refrigerant pumps
− Open Drive and Semi-Hermetic (sealless) centrifugal pumps
Information required to make a pump selection
− Anatomy of a pump curve
− Examples selections
Installation considerations
− Pump suction line, discharge line, Bypass/minimum flow lines, volute
vent line – what is it and why is it need
Operational considerations of a pump
− Cavitation – what is it and how to prevent it
Open drive Close-coupled or belt driven
Single stage
Semi-hermetic (Sealless)
“Canned” pump
Single or multi-staged
Positive Displacement
− Rotary gear, screw or vane
OPEN-DRIVE
Mechanical shaft seal Requires oil reserve Separate air-cooled motor Oil lubricated bearings Impeller trimmed to match
capacity requirement Ability to run dry for short
period of time Nominal 1800 rpm Usually lower initial cost
SEMI-HERMETIC
Sealless design No oil reservoir or oil
maintenance Integral motor Refrigerant cooled motor Hydrodynamic bearings Impeller matched to motor Frost and moisture tolerant Nominal 3600 rpm Usually higher initial cost
System Capacity (US gpm)
Differential Pressure (feet of head or psid)
Refrigerant and Temperature
Net Positive Suction Head available
Voltage and Hertz
Required US GPM =Tons of Refrigeration X Rate of Evaporation (GPM per Ton) X Recirculation Rate
− Multiply the system tonnage by the factor in GPM/Ton table at the required temperature.
− Multiply the resultant GPM by the system recirculation rate (i.e. overfeed rate 3:1, 4:1, etc. + 1) to determine the required GPM of the pump.
− Note: Recirculation Rate is not overfeed rate
GPM = System tons X GPM/ton X Recirc Rate Example: 300 system tons using R-717 at 0F with 4:1 recirc rate
GPM = 300 tons X 0.064 X 4 = 77 GPM
System Capacity Requirement Refrigerant Pump Bypass at Minimum Flow
− Per manufacturers guidelines
Refrigerant Pump Motor Cooling (semi-hermetic)
Total Pump Capacity = System + Bypass + Motor
Additional Consideration
− Future Requirements
Static Losses
− Elevation to Highest Point
Dynamic Losses
− Equipment Pressure Drop
− Valve Pressure Drop
− Pipe Pressure Drop
− Back Pressure Regulators
PSID To Head [FT] Conversion
Head[ft] = PSID x 2.31
________
SPGR
Where SPGR = Specific Gravity
Pressure increase required by pump (inlet-to-outlet)
− PSID = Discharge Pressure - Suction Pressure Discharge Pressure = Static Head + Dynamic Losses
Suction Pressure = Saturated Liquid Pressure
NPSHa is a function of installation 1. Pressure difference above the vapor pressure of the fluid 2. The static height of the fluid above the pump centerline 3. The pressure losses (frictional and form) due to fluid flowing through the suction piping, valves, and the pump’s suction. 4. Heat gains in the piping to the pump suction.
NPSHa
Low Level cut-out
Pump Suction inlet
− NPSHr is a function of pump design at various conditions
Conditions:
− 488 Tons
− 4:1 RERC. Rate
− + 15° F ammonia
− 27 PSID
CAPACITY 488 x .066 x 4 = 128 US GPM
DIFFERENTIAL PRESSURE
27 PSID x 2.31/0.65 = 96 FT. TDH
+15 degree F
Ammonia (SG 0.65)
128 GPM
96 Ft Head
Cavitation
− Due to inadequate NPSHa
Vapor Entrainment
Suction Recirculation
When static pressure of the flowing liquid falls below vapor pressure, bubbles occur, at areas of higher pressure vapor bubbles will suddenly implode.
•
at areas of higher pressure, bubbles will
suddenly implode
Normally at outer diameter / end of vanes
Sounds like gravel in pump Discharge pressure will fluctuate or drop Some evaporators may not be properly cooling Over-temperature thermistors cut-out
− Due to reduction of cooling of semi-hermetic pump motor
material erosion break down of impeller
Temporarily close or partially close pump discharge
line to see if issue goes away Increase of the static pressure on the suction side by
increasing the liquid level Reduce flow requirment to system (HEV settings) Reduce of flow resistance in suction piping ( valves,
filters, diameter of piping etc ) Prevention of turbulences at the inlet of the suction
by a suitable construction ( special impeller design / Inducer )
picutre: inducer in front of impeller
Consider use of flow regulator (semi-hermetic only)
Vapor entrainment
− Bubbles of refrigerant vapor migrating to pump suction
− Usually occurs due to system transients such as start-up conditions, when false loads are terminated (defrost, liquid make-up)
− Prevention: ensure proper recirculator design
Suction recirculation
− Secondary reverse flow occurs within impeller due to insufficient flow through pump
− Ensure Q-Min line is open or set by-pass valve properly
recirculation line
(A) partial flow is deviated with at higher pressure into the motor (B) Pressurised with an auxiliary impeller (C) carried back to the DISCHARGE side of the pump (C). => Some pumps require EXTERNAL PIPING (recirculation line), back to suction vessel for partial flow
(A)
(C)
(B) (C)
Type CAM:
partial flow is devided at high
pressure level from the last
impeller (A) through the motor (B)
and flows back (C) to a lower
pressure level between the stages
(D)
Internal partial flow
No external recirculation piping
needed
Multistage pumps often
consume less Energy compared to
single stage pumps.
Designed for NH3 (Ammonia)
and CO2 Applications
(A)
(B)
(C)
(D)
Pump Suction Line
Pump Discharge
Line
Pump
Pump Vent/Bypass Line
Minimum pump suction line sizing from the accumulator vessel (pump recirculator).
L = 5*DNs Suction pipe downwards
Venting not possible
correct
wrong
Avoid any unnecessary pressure drop in the pump suction
line from valves, strainers, and fittings.
Gas will collect in top portion of volute and must be vented-off Used during pump start-up and prior to servicing Needs to be separate from bypass or suction vent
lines
Volute Vent Valve
Safe guards pumps against insufficient flow
Steps to set properly:
1. Open by-pass valve completely
2. Close discharge stop valve
3. Slowly close by-pass valve until discharge pressure unstable
4. Slowly open by-pass valve until pressure stabilizes
recirculation line