Pump Selection for Laboratory Scale Fluid Handling

As laboratory processes grow in scale and economics drives towards automation, how to automatically move the fluids becomes a design issue with an ever-increasing number of potential answers. The immense variety of lab pump types available can be overwhelming.

Key Challenges:

  • Sorting through the vast offerings of lab pumps on the market is a daunting task.
  • Knowing what information is needed to select the right style of pump.

Recommendations:

  • Look carefully at what you need from your pump. Every type of laboratory pump has its own set of advantages and drawbacks.
  • When you have a strong set of requirements for your pump, it becomes much easier to hone in on a specific pump type.
  • Use the pump manufacturer’s applications services to help you with your final pump selection

INTRODUCTION

The number of pump types available has become overwhelming, and all the manufacturer’s literature focuses on the strengths of the product. It can be difficult to find literature that clearly describes the things to watch out for with each pump type as well as the strengths. Below are some descriptions of pump types that are frequently used in laboratory scale automated fluid handling with some of their disadvantages as well as their advantages. We will look at the following types of pumps that are commonly used with laboratory scale applications:

  1. PLDC’s
  2. Peristaltic
  3. Diaphragm
  4. Syringe
  5. Magnetic Drive Impeller
  6. Gear
  7. Lobe

There are many more pump types available, and each one has specific advantages and disadvantages. This article focuses in on some of the pump types most commonly found in laboratory automation and the information you should know to successfully select a pump type.

best match between process requirements and pump type characteristics

Lab Pump Type Characteristics

1. Pressurized Liquid Dispensing Container (PLDC)

PLDC Pressurized Bottle

Figure 1. Pressurized bottle (image courtesy of Cole-Parmer)

The simple pressurized container is very familiar to most labs. This method does not use a pump or a motor, only a pressurized gas and a suitable container.

There are safety driven limits on the pressures, container volumes, and container types for flammable chemicals used for pressurized dispensing. (Please refer to the container manufacturer’s specifications and the most current version of NFPA 45 for code limitations.)

A pressurizing gas such as air, nitrogen, or argon is connected to the top of the container through one of the cap tubes.  (Note that air should never be used to pressurize a container of flammable liquid.)  A dip tube or a bottom port in the container allows fluid to exit the container at the pressure supplied by the gas. Typically, the pressures are very low, often less than 5 psi. Knowing the pressure rating of the container and properly limiting the pressure supply is critical to the safety of this method.

Special containers designed for the pressurized dispensing of liquids are available for many of the standard solvents and chemicals sensitive to exposure to air. These are referred to as PLDC’s (Pressurized Liquid Dispensing Containers).  These returnable, stainless containers are ordered from the chemical supplier already charged with the chemical. When the containers are empty, they are shipped back to the chemical supplier to be re-used. Using this method, the chemical container is never opened to the atmosphere and has a pressure relief valve that prevents it from exploding if over-pressurized or heated.

Advantages:

  1. Very simple and reliable system with no moving parts.
  2. Pressure can be varied with a pressure regulating device on the pressurization gas supply.
  3. Inert gases such as argon can be used to avoid any reaction with active chemicals.
  4. Very low-cost.
  5. Gives steady, non-pulsing flow.
  6. No electricity is required, which is an advantage around flammable chemicals.
  7. Many hazardous chemicals are available in containers already set up for pressurized dispensing with a built-in dip tube and a separate pressurization port.
Pure Pac II Container Cutaway MIcromatic MacroValve Coupler

Figure 2. Pure-Pac II PLDC with Micromatic Valve Coupler (Reproduced with permission from Sigma-Aldrich Co., LLC)

Cautions

  1. You cannot run a recirculating system or reverse flow with the basic layout of this method. The fluid will not flow back into the container without a pump or a reverse pressurization.
  2. Be very careful of the combination of container type, pressure regulation, pressure relief, and how it is controlled. A glass bottle filled with a flammable or toxic fluid with a 20-psi burst pressure connected to a 2,000-psi nitrogen bottle is an obvious hazard. The dispensing system needs to be set up with safety in mind with respect to the pressure rating of the container, the pressure available, and an over-pressure relief system vented to a safe location.
  3. You must have a source of pressurized gas. If it needs nitrogen or argon and the lab is not piped with the required gas already, dealing with bottled gasses can add some hassle to the operation. Ensure the regulators are appropriate for the pressure range you intend to work in. A 2,500-psi regulator and gauge should have a secondary regulator system if you intend to be working with 5 psi in your PLDC.
  4. If used in an automated system, the emergency stop function should be set up to dump the pressurized gas from the container to prevent extended leakage from occurring. The gas released from the container may container flammable or toxic fumes, so care must be taken with the handling of the released gas. Depending on the quantities and hazards of the fluids, system leak detection tied may need to be installed and be tied in to the emergency stop system.
  5. If using flammable fluids, pay close attention to the grounding requirements and the potential for static electricity generation and discharge throughout your system.

2. Peristaltic Pumps

Peristaltic pumps

Figure 3. Peristaltic Pump (Image courtesy of Cole-Parmer)


peristaltic operation

Figure 4. Peristaltic Operation (image courtesy of Cole-Parmer)

Peristaltic pumps are the top choice for high purity systems.  The pump works on the exterior of a flexible piece of tubing.  This means there is a straight flow path for the fluid, it never leaves the tubing, and the fluid contact piece can be cleaned, sterilized, or replaced easily.

Advantages:

  1. Cleanest flow path of any pump – lowest potential for contamination of the fluid and ability to easily change out the tubing for a clean section if needed.
  2. Positive displacement pump. Knowing the number of cycles of the pump allows you to know how much fluid has passed.
  3. Able to handle high-viscosity fluids
  4. Can be self-priming.
  5. Many well-developed units are out there with built-in controllers.
  6. Flow is easily reversible.

Cautions:

  1. Not only can the tubing be replaced, it MUST be replaced on a regular schedule.
  2. Worn tubing can crack and leak as well as release particulates into the fluid. If you have a hazardous fluid, assume leaks will eventually happen. Leak detection may be needed.
  3. Flow from a standard peristaltic pump has significant pulsation. Different designs are available with higher numbers of rollers, multiple out-of-phase roller sets on parallel tubing, asymmetric roller paths, or pulse dampeners to reduce the pulsation.
  4. Be careful with flow sensors and flow switches. The pulsating flow causes problems with many flow sensors.  Monitoring flow from the pump controller counting revolutions is often more reliable than a traditional flow sensor.

3. Diaphragm Pumps

Air Operated Double Diaphragm Pump

plastic aodd process pump

Figure 5. Plastic AODD Process Pump (image courtesy of SMC Corporation)


aodd

Figure 6. AODD (Image courtesy of SMC Corporation)

The operation of the AODD pump is described below:

aodd pump operation description

Figure 7. AODD pump operation description from www.versamatic.com, adapted from Samtar on Wikipedia

The AODD pump is a favorite for many reasons. It is a very flexible system that gives it a significant list of advantages:

  1. It is air operated. It can operate without any electricity at all. Removing a potential ignition source is ideal when working with flammable fluids.  Most designs are intrinsically safe, meaning they will not generate sparks or enough heat to cause ignition, and can therefore be used with flammable chemicals.
  2. Many chemical pumps do not like pumping air, they rely on the liquid flow to cool bearings. Without liquid, they quickly overheat. The AODD, however, can be pumped dry without damaging it.
  3. The AODD is self-priming. Unlike most impeller type pumps, you do not have to fill the system with liquid to make it start pumping. You can have lines completely full of air leading to your liquid source, and the AODD will handle it no problem if the suction required is within the pump vacuum capacity.
  4. The pump can be deadheaded (attempting to run the pump with the outlet blocked) with no harm to the pump.
  5. The outlet fluid pressure of the AODD is directly proportional (and usually directly equal to) the inlet air pressure. Is 60 psi too much for your process?  Turn your air pressure down to 20 psi!  Do you need more? Turn your air pressure up to 100 psi! (Note that air pressure changes will affect pumping speed as well, and pumps will have limits to the air pressure you may safely apply.)
  6. The speed of the AODD can be modified to some extent by controlling the rate of air flow into the pump with a simple needle valve. Less air flow means the pump will move slower.
  7. The AODD is a positive displacement pump. By counting the strokes, you know how much volume has been moved through the pump. Although it is not quite as accurate as a piston pump or a syringe pump, the accuracy is enough for many applications.
  8. You can start and stop the pump as often as you want. There is no motor to overheat with multiple starting cycles.
  9. The pump has very low fluid shear. Fluid shear is important in many coating mixtures and sensitive chemicals.
  10. The AODD will pass solid particles and slurries through it. Sharp particles will eventually damage the check valves and wear the diaphragms, but it will pump just about anything that can make it through the entrance and exit ports.  One large pump from Wilden is rated to pass 2” diameter solids through it.
  11. There is a huge variety of materials and sizes available in this pump style. Exotic plastics and rubber compounds are available to handle the most corrosive chemicals, and the least expensive materials are available for low-cost applications. AODD’s are available with single stroke volumes down to 5 ml and up to more than 5 liters.
  12. The pump is relatively low-cost depending on the materials selected. In general, because there is no motor it is frequently less expensive than the equivalent motor driven pump. PVDF, PEEK, and 316SS bodies are obviously more expensive than polypropylene or nylon bodies.
  13. The pumps can be set up with automatic shuttling air valves that cycle the pump as fast as it can go, or you can control the air ports and shuttle the pump one cycle at a time for dosing.

However, no pump is perfect! The cautions for using AODD pumps include:

  1. Pulsating flow. The flow is based on the diaphragm’s reversal of direction. Every time it strokes, you get flow. At the end of the stroke, there is no flow. This means a heavy pulsing to your flow. If a high inlet pressure is used with no restriction to the air flow and the outlet is low pressure, the pulsation can be quite violent to the point of shaking fittings loose and cracking plastic tubing supports. The shaking can be carried through the piping around the machine, which is a severe problem for vibration-sensitive processes. Pulsation dampening devices and pressure regulators are sold that do reduce the pulsation and shaking after the pump. The bladder types are large and bulky.  Either type can add significant cost depending on the materials required by your chemicals.
  2. The pulsating flow can cause waves in process baths where they are often not wanted.
  3. The pulsating flow can wreak havoc with flow sensors. Be very careful with specifying a flow sensor on a system using an AODD pump. You may be better off counting the strokes of the pump to know your flow rate.
  4. You cannot reverse the flow direction in the pump.
  5. Although the pump depends on ball check valves to operate, I have often found that the check valves in the pumps will leak and allow backflow when they are turned off. If backflow is a problem, you should consider installing separate check valves outside of the AODD.
  6. Most AODD pumps do not fully drain. There is almost always some liquid left in the pump. If you are servicing an AODD that has been used with hazardous fluids, expect to find fluid inside the pump when you disassemble it. It should only be disassembled in a safe place like a flow hood using PPE if it has been used with hazardous fluids.
  7. The diaphragms and ball check valves do wear out. The more abrasive particles passed through the pump, the quicker it will wear out.
  8. The flow rates can be inconsistent. Changes to inlet/outlet conditions or to the incoming air pressure and flow will have a direct effect on pumping speed.
  9. The AODD pumps are generally noisy.

3.1 Electric Diaphragm Pumps

Electric diaphragm pumps can be motor or solenoid driven. They share most of the characteristics of the AODD pumps above but operate with electricity instead of air.  Many laboratory styles are available with or without controllers. Single and double diaphragm types are available. Single diaphragms have a more noticeable pulsation, but can deliver accurate dosing down into the microliters depending on the pump.

smc lsp solenoid operated single diaphragm pump

Figure 8. SMC LSP Solenoid Operated Single Diaphragm Pump, available from 5 to 200 microliters per shot (image courtesy of SMC Corporation)


masterflex single diaphragm motor driven chemical dosing pump

Figure 9. Masterflex Single Diaphragm Motor Driven Chemical Dosing Pump (image courtesy of Cole-Parmer)

4. Syringe Pumps

Syringe pumps are exactly what they sound like. They typically utilize a standard medical-type glass syringe body that can be removed/replaced from the drive mechanism. Most syringe pumps are screw-driven stages that can push or pull on a standard plunger in the syringe. Glass syringes generally give higher accuracy than a plastic syringe body, and are commonly used.

single syringe pump

Figure 10. Single Syringe pump (image courtesy of Cole-Parmer)


multi syringe pump

Figure 11. Multi-Syringe Pump (Image courtesy of Cole-Parmer)

Advantages:

  1. Useful for tiny flow rates, generally used for microfluidics.
  2. Many of these devices can use multiple sizes of syringes, so different volumes and flow rates are available based on the syringe body diameter.
  3. Many syringe pumps can reverse and pull fluid back into the syringe.
  4. Can get very tight control on flow rates, often less than 1% flow variation.

Cautions:

  1. Standard syringe pumps have flow stability problems at low speeds. Proper syringe size for the flow rate is needed to keep stability.
  2. The volume to be pumped is limited by the volume of the syringe.
  3. Most pumps are driven with a stepper motor. On low-flow applications, motor steps can be seen in the fluid flow.
  4. The pressure is not controlled and can build up if the flow path is restricted. Pressure is limited by the slipping torque of the stepper motor. Be aware of what potential pressure can be generated if the pump is deadheaded.

5. Magnetic Drive Impeller Pumps

Impeller pumps in general are the workhorses of the pumping world. The focus here is on the Magnetic Drive variety (often referred to as Mag Drive) because they have a very important advantage: the fluid path can be completely sealed from the motor, so there is no leak path from the pump head into the motor as seals wear out. With hazardous fluids, this can be a crucial advantage.

mag drive pump

Figure 12. Sealing, standard impeller pump (Images courtesy of Finish Thompson, Inc.)


sealing standard impeller pump

Figure 12b. Mag drive pump (Images courtesy of Finish Thompson, Inc.)

Advantages:

  1. Impeller pumps give a smooth, consistent flow and can have relatively quiet operation.
  2. The motor on a magnetically driven pump is very well protected against leaks, and can even be mounted on the opposite side of a bulkhead from the pump head and the chemical process area.
  3. Available in an almost infinite variety of sizes, shapes, materials, flow, and pressure characteristics. There are specialty impeller pumps designed for an amazing variety of applications.
finish thompson mag drive impeller pump

Figure 13. Finish Thompson Mag Drive Impeller Pump (image courtesy of Finish Thompson, Inc.)

Cautions:

  1. Impeller pumps are not capable of reversing the flow.
  2. High fluid shear at the impeller tips can damage sensitive fluids such as coatings. If you are pumping a viscous coating or a complex chemical, check with the manufacturer of the chemical to ensure there is no shear sensitivity with your fluid.
  3. Impeller pumps can heat fluids, especially if they are run deadheaded or if the fluid is highly viscous.
  4. Cavitation can be a problem. Use your pump distributor to help you avoid this problem. Make sure they understand exactly what fluids you will be pumping and the inlet conditions of the pump.
  5. There are special designs for special cases. If you need one of these characteristics, ask your vendor specifically for it:
    1. “Run Dry” capability. Many designs will destroy themselves if run without liquid as they rely on the passing fluid to cool the bearing sets. There are other pumps like the Finish Thompson shown in Figure 12 that are designed to survive running dry.  If your pump cannot run dry, we recommend protecting it with a switch to ensure fluid flow is occuring.
    2. Suspended Solids. Running solids and grit through a pump may destroy some very quickly, but there are impeller pumps designed specifically to handle suspended solids.
    3. Self-Priming. In order to start a pump that is pulling liquid up from below itself, an impeller turning in air cannot create enough suction to pull water into the pump. Self-priming pumps use a reservoir of liquid that recirculates in order to get the pumping started. This type of pump is not usually applicable to laboratory applications. If you need a pump that does not require priming, other pump types like the gear, diaphragm, or peristaltic pump may be a better choice if your process can handle the pulsating flow.
    4. EX or ATEX rating. Many pumps are available with ratings to handle flammable fluids or operate in a potentially explosive atmosphere. Most use special motors and require special wiring practices to isolate the opportunity for a spark.
    5. Duty cycle. Not all motors are made for constant starting and stopping. Others are rated for this type of use.  The important point is to make sure your pump vendor understands your application. If you are going to be turning the pump on and off constantly, they can often install a different motor on the pump head that can handle the duty cycle.
  6. Fluid can backflow easily through most impeller pumps. Plan on using check valves or normally closed process control valves if backflow through an unpowered pump creates problems.

6. Gear Pumps

There are multiple gear pump designs. They are primarily used for delivering very high pressures or pumping highly viscous fluids. Three types of gear pump designs are shown in Figure 14 below.  In general, gear pumps work by using two inter-meshed gears to pull fluid into an expanding cavity between gears as the teeth separate. When the fluid reaches the point where the gear teeth come together, the mating action of the gear teeth compress the fluid and push it out a port in the housing.

gear pump designs

Figure 14. Gear Pump Designs (image from Wikipedia)

 

gear pump with motor and disassembled view of pump head

Figure 15. Gear Pump with motor, and disassembled view of pump head (images courtesy of Unibloc-Pump, Inc.)

Advantages:

  1. Ideal for pumping high-viscosity fluids that will not flow well through a standard impeller type pump. The oil pump in most automobile engines is a gear pump.
  2. Very compact and are generally smaller than impeller pumps.
  3. They deliver smooth, controlled flow.
  4. Usually self-priming.
  5. Gear pumps are positive displacement, so by knowing the number of revolutions, you will know how much fluid has been pumped (assuming no air is present in the line).
  6. Capable of very high pressures, into the thousands of PSI.
  7. Reversible flow is possible simply by reversing the direction of the motor.

Cautions:

  1. This pump type places a high shear on the fluid, although not as high as impeller pumps when operating at the same pressure delta.
  2. Most cannot run dry for extended periods without damage.
  3. Avoid deadheading (blocking the outlet while running the pump).
  4. Cannot handle suspended solids, slurries, or abrasives.
  5. The gears and the mating housing will wear over time, releasing fine particles into the fluid stream and reducing the performance of the pump. If you have a process that cannot contaminate the fluid in any way, be very careful about gear pumps.
  6. Can heat fluids
  7. Although EX and ATEX rated versions are available, it is expensive due to the need for an explosion proof motor and the associated wiring.
  8. Fine particulates can be generated by the gears meshing with each other over time.

7. Lobe Pumps

Lobe pumps are similar in many ways to gear pumps, but the lobes do not actually contact each other like the gears in a gear pump. The gearing between the lobes happens outside the pump head and two separate shafts extend into the pump head, one for each lobe. Because the shafts are cantilevered, a stiff shaft and a good set of bearings is critical to being able to maintain a tiny clearance between the lobes. The smaller the clearance between the lobes, the less backflow occurs between the lobes and the more efficient the pump is. Lobe pumps can be made in very cleanable configurations.  They are one of the few pumps that lends itself to pharma and food CIP and SIP (Clean In Place and Steam In Place), and so are very well-suited to use in biopharma projects.

lobe pump with controller disassembled pump view

Figure 16. Lobe Pump with controller, and disassembled view of a pump head (images courtesy of Unibloc-Pump, Inc.)

Advantages:

  1. The basic design is relatively free of crevices and places to trap fluid, meaning the pump is easily cleaned compared to most other pump designs. Most can be disassembled for cleaning, or can even be cleaned using pharma CIP or SIP procedures without disassembling the pump at all.
  2. Because the lobes do not contact each other, there is lower probability of fine particulates being generated as can happen in an aging gear pump.
  3. The pump design is low shear by nature and is very gentle on the fluid.
  4. The pump can pass solids.
  5. Lobe pumps deliver smooth, controlled flow.
  6. Lobe pumps are capable of up to 90 psi pressure rise across the pump.
  7. Reversible flow is possible simply by reversing the direction of the motor.
  8. Most designs can run dry for at least a period of time and will handle some air or gas bubbles coming through the system.

Cautions:

  1. Self-priming ability is limited.
  2. Low viscosity fluids will have a higher leakage rate between the lobes, meaning the pump efficiency decreases with a lower viscosity fluid.
  3. Avoid deadheading (blocking the outlet while running the pump).
  4. Can heat fluids if deadheaded, but it is more gentle than a gear pump.
  5. Although EX and ATEX rated versions are available, it is expensive due to the need for an explosion proof motor and the associated wiring.

Conclusion

Finding the best lab pump type for your particular application requires a specific knowledge of your process and the fluid.  To aid in the selection of the right pump for your project, start by finding answers to the following questions:

1. What flow rate do you need?
2. What is the viscosity of the fluid?
3. What is the head pressure you will pump against?
4. What is the head pressure available at the pump inlet?
5. What is the vapor pressure of the fluid?
6. What is the temperature of the fluid, and is the fluid sensitive to heating?
7. Do you need the pump to self-prime or handle gas bubbles?
8. Do you need to reverse flow?
9. Is the fluid flammable, corrosive, or have specific material contact needs?
10. Is the fluid shear sensitive?
11. What pressures can you work with?
12. Is the process sensitive to contamination?
13. Do you have solids in your liquid?  If so, what size and what are the solids?
14. Do you care about pulsation or noise?
15. How accurately do you need to control flow rate or volume?
16. Are you passing the fluid through once, or recirculating?

In addition, having a Process and Instrumentation Diagram (P&ID) of your proposed system will help you think through the needs of the system and better communicate the process requirements for the pump.  Utilize fluid handling experts to review your information to ensure you get the pump that best matches the process needs.

Andrews-Cooper draws upon years of experience designing and building laboratory scale automated fluid handling systems.

Give us a call to discuss your application and challenges!

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