10.3 Positive Displacement & Chemical Metering Pumps

Positive Displacement Principles vs. Centrifugal Mechanics

In water treatment, liquid transport falls into two distinct hydraulic categories: dynamic (centrifugal) pumps and positive displacement (PD) pumps. While centrifugal pumps impart kinetic velocity to water using an open spinning impeller, positive displacement pumps operate by physically trapping a fixed volume of liquid within a confined cavity and mechanically forcing that fluid into the discharge pipe.

  CENTRIFUGAL PUMP BEHAVIOR                    POSITIVE DISPLACEMENT PUMP BEHAVIOR
  Head (ft)                                    Pressure (psi)
     |                                            |
     |---\                                        |         | (Nearly Vertical Curve)
     |    \                                       |         | Flow is constant
     |     \ (Sloping Curve)                      |         | regardless of head!
     |      \ Flow decreases as head rises        |         | Danger of overpressure
     +---------\-------> Flow (Q)                 +---------|---------> Flow (Q)

The fundamental operational differences between dynamic and positive displacement pumps dictate their distinct roles in a water utility:

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Reciprocating Chemical Metering Pumps

Reciprocating positive displacement pumps utilize a back-and-forth reciprocating mechanism to drive chemical feed:

                    [ Discharge Check Valve (Ball Rises) ] ---> To Chemical Injection Point
                                     ^
                                     |
    [ Drive Cam / Motor ] ---> [ Diaphragm Cavity ]
                                     ^
                                     |
                     [ Suction Check Valve (Ball Seats) ] <--- From Chemical Day Tank

1. Diaphragm Metering Pumps

The most widely used chemical feed pump in water treatment. A flexible elastomer diaphragm (typically constructed of PTFE/Teflon face bonded to an EPDM backing) isolates the chemical from the drive mechanism:

• Mechanically Actuated Diaphragm: The diaphragm is physically bolted to a reciprocating mechanical push-rod driven by an electric motor and eccentric cam. Simple and economical, but mechanical stresses limit discharge pressures to under 100 to 150 psi.
• Hydraulically Actuated Diaphragm: The reciprocating plunger displaces an internal hydraulic fluid reservoir (oil), which evenly flexes the Teflon diaphragm. Because hydraulic pressure balances equally across both sides of the diaphragm, mechanical stress is eliminated, allowing discharge pressures exceeding 500 to 3,000 psi while providing exceptional diaphragm life.

2. Ball Check Valve Operation

Diaphragm pumps rely entirely on gravity- and differential-pressure-assisted ball check valves located at the suction and discharge ports:

• Suction Stroke: The diaphragm retracts, expanding cavity volume. Negative pressure lifts the suction ball off its seat, drawing liquid in, while the discharge ball is held sealed against its seat by line pressure.
• Discharge Stroke: The diaphragm pushes forward, compressing the cavity. Positive pressure forces the suction ball down tightly against its seat, while unseating the discharge ball to eject liquid into the line.
• Valve Materials: Ball check valves feature high-density ceramic, Hastelloy, or PTFE balls mating with precision Viton or PVDF seats. Clogging or particulate lodging on valve seats causes immediate loss of prime and feed failure.

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Rotary Positive Displacement Pumps

Rotary PD pumps utilize rotating elements (gears, lobes, rotors, or rollers) to trap fluid at the suction port and transport it around the casing periphery to the discharge port.

     PERISTALTIC HOSE PUMP                     PROGRESSIVE CAVITY PUMP
        [Roller / Shoe]                              (Single Helical Rotor in Double Helical Stator)
         +-----------+                           
         |  (====)   |                           +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
  Inlet  | (      )  | Discharge                 |==== Rotor: Tool Steel (Helical)=|
  ------>| [ Hose ]  |--------->                 |~~~~ Stator: NBR Elastomer ~~~~~~|
         | (      )  |                           +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
         |  (====)   |                           Progressing Sealed Cavities Move Solids Smoothly
         +-----------+                           

1. Peristaltic (Hose) Metering Pumps

Peristaltic pumps utilize rotating rollers or shoes that compress a reinforced elastomeric tube (Norprene, Viton, or CSM rubber) against a curved housing. As the roller advances, the hose behind it recovers its round shape, drawing chemical in via vacuum, while the leading roller pushes trapped fluid out:

• Vapor-Lock Immunity: The premier choice for feeding sodium hypochlorite ($NaOCl$) and hydrogen peroxide. Liquid sodium hypochlorite naturally outgasses chlorine and oxygen bubbles in suction lines. Diaphragm pumps "gas bind" (vapor lock) because the gas bubble cushions diaphragm movement, preventing check balls from moving. Peristaltic pumps sweep gas bubbles through the hose without loss of prime.
• Seal-Less Operation: Fluid never contacts mechanical seals, packings, or check valves; the hose is the sole wear component and is replaced on a scheduled runtime basis (typically every 2,000 to 4,000 operating hours).

2. Progressive Cavity Pumps (Moyno Pumps)

Progressive cavity pumps feature a single helical chrome-plated tool steel rotor turning eccentrically inside a double-threaded elastomeric stator (often Buna-N or nitrile rubber):

• Non-Pulsating, Low-Shear Flow: The rotor and stator form a continuous sequence of sealed cavities that progress axially from suction to discharge. This delivers an exceptionally smooth, pulseless flow stream.
• Polymer and Sludge Applications: Progressive cavity pumps are mandatory for feeding high-molecular-weight long-chain organic polymers (polyelectrolytes). High-shear centrifugal impellers slice polymer molecular chains, ruining coagulation efficacy; low-shear progressive cavity pumps preserve polymer chain integrity. They also excel in pumping thick clarifier sludge (4% to 15% solids).
• Mandatory Run-Dry Protection: Because the elastomer stator relies entirely on the pumped fluid for boundary lubrication, running a progressive cavity pump dry for even 30 to 60 seconds will overheat and scorch the stator elastomer, causing catastrophic failure. Systems incorporate thermal sensors or suction flow switches that instantly de-energize the motor upon loss of liquid.

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Chemical Metering Controls and the 20–90 Operating Rule

Positive displacement metering pumps provide dual manual or automated controls to regulate chemical dose:

1. Stroke Length Adjustment: A mechanical micrometer dial adjusts the physical amplitude of diaphragm or plunger deflection (adjusting displaced volume per stroke). Stroke length should only be adjusted while the pump is running to prevent internal linkage binding.
2. Stroke Frequency (Speed) Adjustment: An electronic pulse circuit or Variable Frequency Drive (VFD) alters motor speed (adjusting strokes per minute, SPM).

                [ ACCURACY PROFILE: THE 20% TO 90% RULE ]
   0% ------------- 20% =============================== 90% ------------ 100%
   [ POOR ACCURACY ]    [ OPTIMAL OPERATING WINDOW ]    [ ACCELERATED WEAR ]
   Inertia/Slip Error   Linear, Precise Delivery        Diaphragm Fatigue / 
   Erratic Dosing       Class II Exam Standard          Valve Floating

The Golden 20% to 90% Operating Rule:
For compliant chemical dosing, operators must maintain both stroke length and stroke speed between 20% and 90% of their maximum ratings.

• Below 20% Stroke/Speed: Hydraulic inertia, check valve seating latency, and diaphragm dead-space compressibility produce severe metering errors exceeding $\pm 15%\text{ to } 20%$.
• Above 90% Stroke/Speed: High-frequency cycling accelerates diaphragm mechanical fatigue, causes valve ball chatter, and risks motor overheating.

Drawdown Cylinder Calibration Procedure

Chemical feed rates must be verified volumetrically using a graduated drawdown calibration cylinder installed on the suction line:

  1. Isolate the main chemical storage tank valve.
  2. Open the drawdown cylinder isolation valve to feed from the graduated tube.
  3. Operate the metering pump and record milliliters (mL) consumed over exactly 60 seconds using a stopwatch.
  4. Calculate delivery rate:

$\text{Feed Rate (mL/min)} = \frac{\text{Volume Consumed (mL)}}{\text{Time (minutes)}}$

$\text{Feed Rate (gal/day)} = \frac{\text{Feed Rate (mL/min)} \times 1,440 \text{ min/day}}{3,785.4 \text{ mL/gal}}$

$\text{Chemical Feed (lb/day)} = \text{Feed Rate (gal/day)} \times 8.34 \text{ lb/gal} \times \text{Specific Gravity}$

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Critical Discharge Piping Safety Appurtenances

Because positive displacement pumps force fluid forward with virtually unstoppable hydraulic power, chemical feed manifolds must incorporate strict safety appurtenances:

                                  +---[ Pressure Relief Valve (PRV) ]---> Return to Tank
                                  |    (Set 10-15 psi above line)
    [ Chemical Metering Pump ] ---+ 
                                  |---> [ Pulsation Dampener ] (Nitrogen Bladder)
                                  |---> [ Pressure Gauge / Isolator ]
                                  |---> [ Backpressure Valve (BPV) ] (Maintains 10-20 psi)
                                  v
                 To Water Main Injection Quill

1. Pressure Relief Valve (PRV): An ASME-rated, spring-loaded diaphragm valve installed immediately downstream of the pump discharge, upstream of all isolation valves. The PRV discharge must be piped with clear rigid pipe back to the chemical day tank or storage reservoir. If an operator inadvertently starts the pump with the discharge isolation valve closed (dead-heading), the PRV opens instantly at a set pressure (typically 10 to 15 psi above normal discharge line pressure), relieving excess fluid and preventing exploded piping or destroyed pump heads.
2. Backpressure Valve (BPV): Installed downstream of the PRV, maintaining a constant positive backpressure (10 to 20 psi) on the metering pump discharge check valve. The BPV prevents chemical siphoning when feeding into a line under vacuum or low pressure (e.g., clearwell inlet), and ensures crisp, consistent seating of pump check balls.
3. Pulsation Dampener: A hydropneumatic vessel containing an elastomeric bladder pre-charged with dry nitrogen or compressed air to approximately 70% to 80% of system operating pressure. As reciprocating pumps pulse fluid forward, the bladder compresses during the discharge stroke and expands during the suction stroke. This absorbs shock waves, converts intermittent pulsing into a continuous laminar stream, and protects piping joints and flowmeters from fatigue failure.
4. Corporation Stop and Injection Quill: The injection quill extends into the center third of the process water main, ensuring chemical is dispersed into high-velocity turbulent flow rather than trickling down the pipe wall where it causes localized chemical corrosion.