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Lecture 12: Liquid Circulation Systems and Pumps

Pumps / Compressors

· Centrifugal Pumps - Use centrifugal force to push the fluid through the outlet.

· Metering Pumps - Bellows, diaphragm, peristaltic, piston, and syringe pumps are all metering pumps that pull the fluid through the inlet valve into a chamber, close the inlet valve, and then push the fluid through the outlet valve.

· Positive Displacement Pumps - Bellows, double-diaphragm, flexible impeller, gear, oscillating, piston, progressing cavity, rotary lobe, rotary vane, and peristaltic pumps have a fixed cavity that the fluid is pushed through by rollers, gears, or impeller. As the fluid is pushed through, it leaves a void or vacuum which pulls in more fluid.

Pump Types and Definitions

Bellows - These pumps move fluid through a reciprocating bellows cavity that is coupled to a driving rod. Pumps are found in the "Metering" and "Positive Displacement" pump sections.
Centrifugal - A rotating vanned disk attached to a drive shaft moves fluid without pulsation as it spins. The outlet can be restricted without damaging the pump. Pump flow is not metered.
Diaphragm - Pulsations of one or two flexible diaphragms displace liquid while check valves control the direction of the fluid flow.
Flexible Liner—The outer surface of an inert liner and inner surface of a rotating body block create a fluid channel used to gently pump fluids without pulsation
Flexible Impeller - Elastomeric impeller traps fluid between the impeller blades and a molded housing that sweeps fluid through the pump housing.
Gear - Fluid is trapped between the teeth of two or three rotating gears. Gear pumps are good for high system-pressure applications and are often magnetically driven. Flow can be metered. Pump should not be stalled and requires outlet protection (relief / bypass valve)
Peristaltic (tubing)—Fluid only contacts the tubing—rollers of a motor-driven pump head push the fluid along the tubing as they rotate. Noncontaminating and easy to clean.
Piston—Rotating pistons of varying stroke lengths pump fluids through check valves. Good for high-pressure applications
Rotary Vane—Operate like flexible impeller pumps but use an impeller made of a rigid material—useful for high-pressure or low-shear applications
Syringe—Infusion or withdrawal syringe pumps provide high pressure and high accuracy for applications such as HPLC

Liquid Pump Terminology

Bypass Valve (relief) Protects positive displacement pumps (such as gear pump) http://www.micropump.com/tech_tips/relief_valves.asp

Cavitation Process in which small bubbles are formed and implode violently. http://www.micropump.com/tech_tips/cavitation.asp

Continuous Duty “typically” = 20,000 hrs. Often requires brushless DC motors for space application. Some pumps cannot be run continuously (only intermittent duty). Brush motor, gear pump: 2000-3000 hrs, maybe 6000 hrs., brushless DC motor 20,000 hrs. electronics 100,000 hrs. (1 year = 8760 hrs.). Pump life estimates: http://www.micropump.com/tech_tips/life_estimates.asp

Dead Head The ability of a pump to continue running without damage when discharge is closed off. Only recommended for centrifugal pumps.

Efficiency energy conversion: fluid energy / electric energy = p * V(dot) / P el. Example: 3 psi * 500 ml/min / 3 watt = 0.057 = 5.7%:

Flooded Suction Liquid flows to pump inlet from an elevated source by means of gravity. Recommended for centrifugal pump installations.

Flow A measure of the liquid volume capacity of a pump. Given in gallons per hour (GPH), gallons per minute (GPM), liters per minute (L/min), or milliliters per minute (mL/min).

Fluids Include liquids, gases, and mixtures of liquids, solids, and gases. In this catalog, the terms fluid and liquid are both used to mean a pure liquid or a liquid mixed with gases or solids that acts essentially like a liquid in pumping applications.

Head A measure of pressure, expressed in feet of head for centrifugal pumps. Indicates the height of a column of water being moved by the pump (without friction losses).

Magnetically coupled eliminates dynamic (rotating) shaft seal. Electric motor rotates a magnet. A magnet connected to the pump (impeller, gear) is magnetically coupled through the sealed enclosure (static seal). If pressure exceeds a certain limit, magnets will decouple (requires stop to re-couple).

http://www.micropump.com/tech_tips/magnetic_drives.asp

Pressure The force exerted on the walls of a tank, pipe, etc., by a liquid. Normally measured in pounds per square inch (psi).

Prime Charge of liquid required to begin pumping action when liquid source is lower than pump. Held in pump by a foot valve on the intake line or by a valve or chamber within the pump.

Pulsating Flow

Seals Devices mounted in the pump housing and/or on the pump shaft that prevent leakage of liquid from the pump.

Self-Priming Pumps that draw liquid up from below pump inlet (suction lift), as opposed to pumps requiring flooded suction.

Static Discharge Head Maximum vertical distance (in feet) from pump to point of discharge with no flow.

Strainer A device installed in the inlet of a pump to prevent foreign particles from damaging the internal parts.

Total Head Sum of discharge head, suction lift, and friction loss.

Valves: Bypass Valve Internal to many pump heads that allow fluid to be recirculated if a given pressure limit is exceeded.

Check Valve Allows liquid to flow in one direction only. Generally used in discharge line to prevent reverse flow.

Foot Valve A type of check valve with a built-in strainer. Used at point of liquid intake to retain liquid in system, preventing loss of prime when liquid source is lower than pump.

Relief Valve Used at the discharge of a positive displacement pump. An adjustable, spring-loaded valve opens when a preset pressure is reached. Used to prevent excessive pressure buildup that could damage the pump or motor.

Viscosity The "thickness" of a liquid or its ability to flow. Most liquids decrease in viscosity and flow more easily as they get warmer.

Tips to Keep in Mind:

· Restricting the inlet port size and the inlet pipe ID will cause cavitation and damage the pump.

· It is best to have a straight run of pipe leading into the pump inlet.

Pump Selection

http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=SelectingLiqPumps.htm

Use the guide below to help you select the best type of pump for your application. This information is intended as a general guideline and will not hold true for all pumps within a classification; check individual pump specifications on the given product pages for complete details.

Pump type	Max flow ranges		Max pressure	Self-priming	Pulseless flow	Fluid viscosity	Particulate matter	Run dry	Advantages
Pump type	GPM	L/min	Max pressure	Self-priming	Pulseless flow	Fluid viscosity	Particulate matter	Run dry	Advantages
Bellows	0.008 to 26.4	0.03 to 100	Up to 73 psi	Good	Poor	Medium	Yes	Yes	Can pump liquids or gases
Centrifugal	2.3 to 1200	8.7 to 4542	Up to 275 psi	Poor	Excellent	Light	No	No	Fluid transfer at high flow rates and low pressures
Diaphragm	0.003 to 5.2	0.01 to 19.7	Up to 300 psi	Good	Poor	Medium	No	Yes	High-accuracy; for applications such as pH/ORP control
Double- diaphragm	1.0 to 4.0	3.79 to 15.1	Up to 95 psi	Excellent	Fair	Medium	Yes	Yes	Use for viscous or particulate-laden fluids
Flexible impeller	3.8 to 50.0	14.4 to 189	Up to 60 psi	Excellent	Excellent	Light	No	No	Low-cost utility pump
Flexible liner	1.0 to 10.0	3.8 to 37.8	Up to 50 psi	Excellent	Excellent	Medium	Yes	Yes	Gentle pumping action uses no seals, pulseless, can run dry
Gear	0.006 to 74.0	0.026 to 280	Up to 1500 psi	Poor	Excellent	Medium	No	No	Pulseless flow at high pressures
Ismatec™ peristaltic	0.00002 to 1.43	0.00008 to 5.4	Up to 22 psi	Excellent	Fair	Heavy	Yes	Yes	Noncontaminating; high accuracy; available in a wide variety of tubing materials;
Manostat® peristaltic	0.0008 to 1.3	0.003 to 5.0	Up to 25 psi	Excellent	Fair	Heavy	Yes	Yes	Noncontaminating; available in a wide variety of pump materials
Masterflex peristaltic	0.0000034 to 12.0	0.013 45	Up to 100 psi	Excellent	Fair	Heavy	Yes	Yes	Noncontaminating; available in a wide variety of pump materials
Nutating disc	0.25 to 1.0	0.95 to 3.8	Up to 15 psi	Good	Poor	Medium	Yes	Yes	Teflon® wetted parts; positive displacement
Piston	0.004 to 107	0.015 to 405	Up to 5000 psi	Good	Poor	Medium	No	Yes	Highest pressure and accuracy; ideal for HPLC applications
Progressing cavity	0.5 to 13	1.9 to 49	Up to 100 psi	Fair	Excellent	Very heavy	Yes	No	Pulseless flow for highly viscous or particulate-laden fluids
Rotary vane	0.75 to 4.3	2.8 to 16.3	Up to 240 psi	Fair	Very good	Light	No	No	High-pressure capabilities; low shear
Syringe	0.002 to 0.04	0.008 to 0.15	Up to 40 psi	N/A	Excellent	Light	No	Yes	Low flow rates at high pressures

Pressure Definitions

· Psid (Pressure Differential): a measure of the difference between two pressures.

· Psig (Gauge Pressure): a measure of pressure in psi that is referenced to ambient pressure

o special case of differential pressure, where pressure difference between system and ambient is measured.

· Psia (Absolute Pressure): a measure of pressure in psi that is referenced to zero absolute pressure.

o special case of differential, where pressure difference between system and a vacuum reference within sensor is measured.

· MOP: Maximum operating pressure. This pressure is expected to occur during normal / nominal operation. For example, backpressure of circulation system at a nominal pump speed.

· MDP: Maximum design pressure. This pressure can occur under credible combinations of extreme conditions (i.e., highest temperature and stalled pump, or highest temperature and space vacuum exposure, or MOP + vacuum exposure). Include tolerances on settings of regulators, relief valves, pump speed etc.

Safety Hazards

· Overpressurization:

o Possible Cause: blocked flow (frozen), particulates, kinked line, pump speed (voltage)

o Possible Controls:

§ Relief / bypass valves

§ Current limit on pump

§ Pressure sensor control / pressure switch

· Leakage:

o Possible Cause:

§ Assembly error, faulty components, materials incompatibility,

§ structural failure due to freezing / over-temperature / rupture of lines.

o Testing:

§ Leak (pressure decay – how long ?, visualization, Helium sniffer) – 1.5 x MDP.

§ Proof test (1.5 x MDP).

§ Note: pressure testing with gas can be dangerous if tested to failure, due to stored energy in compressed gas. Test with liquid (non-compressible. Hydro-static testing) and/or submerged in water (pressure tank testing).

· Pressure Rating:

o Ultimate pressure rating (failure: leakage, or structural failure)

o Maximum Allowable Pressure (P <= MDP)

o Factor of Safety [at MDP to ultimate pressure] = ultimate pressure / MDP

§ Minimum FOS required: typically 4x for lines, fittings, tanks often require only 2.5x, but rigorous testing

o Margin of Safety [at MDP to ultimate pressure] = [FoS(actual) - FoS(required)] = UP/MDP - FoS(required)

§ MS should be positive (>0).

Examples of Implementation

Coolant Circulation Pump (left): positive displacement gear pump (»8,000 hrs) with brushless DC motor (20,000 hrs., 3-5 Watt) on left with pressure sense ports. Pump motor is fitted with a microprocessor-based time counter for lifetime tracking. Bellows-based expansion vessel (accumulator) is seen on upper right. Fluid components are insulated with closed-cell foam to avoid condensation while running below cabin dew point (not shown).

Pump performance as a function of pressure, and pressure limiting safety bypass valve. A Raychem Polyswitchä resettable fuse (I_hold=200mA, I_trip=400mA) and the mechanical bypass valve ensure that the system pressure will not exceed »200 kPa (30 psi_d). During nominal operation, the pump performance is also monitored by the computer (P_in, P_out, DP=P_out-P_in).

The current bellows-based expansion vessel exhibits a volume-dependent spring constant. Typically, the positive travel is limited by the second level of containment (Polycarbonate body) at 6 ml positive expansion. To some limited extent, the expansion vessel also serves as a reservoir to compensate for water losses (diffusion through flexible tubing).

Pressure Dynamics

The payload computer monitors the inlet and outlet pressure sensor continuously, but stores data only once a minute. The data in the Figure on left shows pressure fluctuation as a function of external pressure swings (shown simultaneously on both P_in and P_out), but also exhibits pressure dynamics that are not fully understood.

Pressure curves for 2 months operation on ISS.9A (launch STS-112, landing STS-113). The gauge pressures show diurnal changes due to heat loads changing daily (day/night cycle), as well as differential pressure decay over time. The ‘sudden’ simultaneous changes of the gauge pressures at certain mission increments are not fully understood (i.e., they do not coincide with cabin pressure changes or large thermal changes).