selection and operational considerations

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