Diaphragm Pump: Overview, Selection, and Operation

I. Overview of Diaphragm Pumps:

Diaphragm pumps consist of an actuator and valves, powered by compressed air. They are capable of handling a wide range of fluids, including corrosive, abrasive, high-viscosity, volatile, flammable, and highly toxic substances. These pumps find applications in specialized settings where conventional pumps fall short.

Diaphragm pumps come with two types of actuators: pneumatic and electric, known as QBY-type pneumatic diaphragm pumps and DBY-type electric diaphragm pumps, respectively. The main body (valve body) of a diaphragm pump can be constructed from four materials: plastic, aluminum alloy, cast iron, and stainless steel, selected based on the requirements of the fluid or gas medium. The internal diaphragm material varies according to the type of liquid medium, which may include nitrile rubber, chloroprene rubber, fluorine rubber, polyvinylidene fluoride, and polytetrafluoroethylene. Therefore, specifying the medium type is crucial when placing an order.

II. Specific Selection Criteria for Diaphragm Pumps:

1. Selection of Operating Mode for Diaphragm Pumps:
   • The operating mode of a diaphragm pump is relevant only when choosing a pneumatic actuator. It is determined by the combination of the actuator’s forward and reverse actions with the valve’s forward and reverse actions. This combination yields four possible modes: positive-positive (air-closing), positive-reverse (air-opening), reverse-positive (air-opening), and reverse-reverse (air-closing), resulting in two types of diaphragm pump operations: air-closing and air-opening.
   • Considerations for selecting the operating mode:
     • Process safety in production.
     • Characteristics of the medium.
     • Ensuring minimal economic losses while guaranteeing product quality.
2. Selection of Flow Characteristics for Diaphragm Pumps:
   • Flow characteristics refer to the relationship between the relative flow through the valve and the displacement (relative opening of the valve). There are four ideal flow characteristics: linear, equal percentage (logarithmic), parabolic, and quick-opening. Commonly used ideal flow characteristics are linear, equal percentage (logarithmic), and quick-opening. Parabolic flow characteristics fall between linear and equal percentage, with equal percentage often substituting for them. Quick-opening characteristics are primarily used in two-position control and program control. Therefore, choosing flow characteristics for diaphragm pumps essentially means selecting between linear and equal percentage flow characteristics.
   • Selection of flow characteristics can be theoretically calculated, but the methods and equations involved are complex. Empirical guidelines are commonly used, considering:
     • Analysis and selection based on the regulation quality of the control system.
     • Consideration of process piping conditions.
     • Analysis of load variations.
   • Choosing the flow characteristics of a diaphragm pump enables the determination of the valve core’s shape and structure based on its flow characteristics. However, for valves like diaphragm valves and butterfly valves, due to their structural features, it is not possible to achieve the required flow characteristics by changing the valve core’s surface shape. In such cases, it can be accomplished by altering the feedback cam shape of the valve positioner.
3. Selection of Diameter for Diaphragm Pumps:
   • The selection and determination of the diaphragm pump’s diameter primarily depend on the valve’s flow capacity, known as cv. In instrument design and selection for various engineering projects, diaphragm pumps undergo cv calculations, and a diaphragm pump design manual is provided. The process from cv calculation for diaphragm pumps to valve diameter determination generally involves the following steps:
     • Determining the calculated flow rate. Existing production capacity, equipment load, and the condition of the medium determine the maximum (qmax) and minimum (qmin) calculated flow rates.
     • Determining the pressure differential before and after the valve. Selecting s (resistance coefficient) based on the chosen valve flow characteristic and system characteristics, then determining the calculated pressure differential.
     • Calculating cv. Choosing appropriate calculation formulas and charts based on the adjusted medium to obtain cmax and cmin.
     • Selecting cv. Based on cmax, choose the next higher grade c in the selected product standard series that is closest to cmax.
     • Checking the diaphragm pump’s aperture. The aperture at maximum calculated flow rate is generally required to be ≤90%, and at minimum calculated flow rate, it should be ≥10%.
     • Checking the actual adjustable ratio of the diaphragm pump. Generally, an actual adjustable ratio of ≥10 is required.
     • Determining the valve seat diameter and nominal diameter. After verification, determine it based on c.
4. Selection of Diaphragm Pump Type:
   • Selection of Valve Body Type for Diaphragm Pumps:
     • The selection of the valve body is the most critical step in diaphragm pump selection. There are various types of diaphragm pump valve bodies, with ten common ones including straight-through single seat, straight-through double seat, angular, diaphragm, low-flow, three-way, eccentric rotation, butterfly, sleeve, and spherical. Before choosing a valve, a careful analysis of the medium in the control process, process conditions, and parameters must be conducted, with sufficient data collected to understand the system’s requirements for the diaphragm pump. Based on the collected data, the type of valve to be used is determined. When making specific selections, consider the following factors:
       • A. Valve core shape and structure should be considered primarily based on selected flow characteristics and factors like unbalanced forces.
       • B. Wear resistance: In cases where the fluid medium contains high concentrations of abrasive particles in suspension, the valve core and valve seat mating surfaces will experience severe friction during each closure. Therefore, the valve’s flow path should be smooth, and the internal materials of the valve should be hard.
       • C. Corrosion resistance: In the presence of corrosive media, if the valve can still meet the regulation function, it is advisable to choose a structurally simple valve.
   • Selection of Actuator for Diaphragm Pumps:
     • Considerations for Output Force:
       • Regardless of the type of actuator, its output force is the effective force used to overcome loads (primarily referring to unbalanced forces and moments, along with the effects of friction, sealing force, gravity, and other related forces). Therefore, to ensure the normal operation of a diaphragm pump, the accompanying actuator should generate sufficient output force to overcome various resistances, ensuring high-level sealing and valve opening.
     • Determination of Actuator Type:
       • Once the output force of the actuator is determined, select the appropriate actuator based on the process environment requirements. In cases where explosion protection is required on-site, use a pneumatic actuator with an explosion-proof type junction box; an electric actuator should not be chosen. If explosion protection is not required, both pneumatic and electric actuators are options. However, for energy conservation, electric actuators should be preferred whenever possible. While hydraulic actuators are not as widely used as pneumatic and electric actuators, they offer high precision adjustment, fast and smooth action, and may be necessary in certain situations to achieve optimal regulation, such as speed control of transparent machines in power plants or temperature control of catalyst reactors in refining plants.

5. Operating Precautions for Diaphragm Pumps:

• Starting and Stopping Load Operations of Diaphragm Pumps:
  • Diaphragm pumps are driven by variable-frequency motors. If the starting or stopping load is too large or too small, it can easily damage the motor. To avoid such situations, it is mandated to keep the starting and stopping loads controlled at around 30%. Additionally, measures have been implemented in the instrumentation to ensure that both load addition and reduction require a specific duration to complete, providing effective maintenance for the frequency converter from a process perspective.
• Operation of Instrument Air and Lubricating Oil Pressure Systems for Diaphragm Pumps:
  • The instrument air and lubricating oil pressure of diaphragm pumps are interlocked with the main motor. The filling and draining valves for the propellant fluid are air-locked valves, requiring a constant air pressure to be maintained. The minimum action pressure for both valves should not be less than 0.4mpa. Therefore, the instrument air pressure is generally maintained above 0.15mpa, even when the pump is stopped, to avoid insufficient or excess liquid in the propellant fluid chamber. The air supplied to the valves is filtered and atomized with oil.
  • During operation, the lubricating oil pressure of diaphragm pumps should not be lower than the pressure interlock value (0.115mpa). Additionally, the pressure difference across the outlet filter for the lubricating oil should not be too high, and the steel ball in the sight glass for the lubricating oil flow should be at the upper limit position. When adjusting this flow, the adjustment range should not be too large, to prevent a rapid pressure drop due to excessive instantaneous flow, which may cause interlock tripping.

By implementing these process improvements and proficiently mastering the key operational points, diaphragm pumps can operate in a stable process environment. Over a year of operation, there was only one trip due to electrical reasons, demonstrating prolonged, safe, and efficient operation.

III. Conclusion:

The selection of diaphragm pumps involves careful considerations encompassing valve core shape, wear resistance, corrosion resistance, medium temperature and pressure, and the prevention of flashing and cavitation. Additionally, the type of valve body and actuator, as well as operational precautions, play crucial roles in ensuring the successful and efficient operation of diaphragm pumps in various applications.