A Form 2 force pump is a double-acting reciprocating piston pump where both faces of the piston do useful work — one side draws fluid through a suction valve while the other side discharges through a delivery valve, then the roles swap on the return stroke. You see this layout in the classic Tangye and Worthington duplex pumps still used in heritage waterworks and shipboard ballast service. The purpose is to deliver near-continuous flow from a single piston, doubling output per revolution compared with a single-acting Form 1 pump. A well-built unit holds pressures of 5 to 50 bar reliably for decades.
Force Pump (Form 2) Interactive Calculator
Vary bore, rod size, stroke, speed, and volumetric efficiency to see double-acting pump flow and displacement update live.
Equation Used
The double-acting displacement per revolution is the sum of both piston faces times stroke, minus the piston-rod area on the rod side. Volumetric efficiency eta_v reduces theoretical flow to estimated delivered flow.
FIRGELLI Automations - Interactive Mechanism Calculators.
- Double-acting piston pump with one piston rod passing through one side.
- One forward and one return delivery stroke occur per revolution.
- Liquid is treated as incompressible; leakage and valve losses are included only through volumetric efficiency.
- Inputs use mm and rpm; calculation converts internally to SI units.
The Force Pump (form 2) in Action
The Form 2 force pump puts a piston inside a closed cylinder with valve ports at both ends. On the forward stroke the piston pushes fluid out the front-end delivery valve while simultaneously pulling fluid in through the back-end suction valve. On the return stroke the four valves reverse roles. This is what double-acting means — every stroke is a working stroke, and you get two delivery pulses per revolution instead of one.
The piston rod must pass through a packed gland on the back end of the cylinder, which is the engineering price you pay for the second working face. That gland is the most common failure point on the whole pump. If the packing nut is over-tightened the rod scores and you lose volumetric efficiency on the suction side because air leaks past the gland during the back-chamber suction stroke. Under-tighten it and the pump weeps fluid down the rod. The rule we give customers rebuilding heritage pumps — tighten the gland nut by hand until rotation just becomes stiff, then back off a sixth of a turn.
The four valves — two suction, two delivery — are typically disc or ball clacks seating on bronze or gunmetal seats. Valve lift must be limited to roughly 25% of the seat bore. Lift more than that and the valve slams hard enough to crack the seat over a few thousand cycles. Lift less and you choke the flow and lose head. The air vessel sitting on the delivery side absorbs the pressure pulses between strokes — without it, every reversal hammers the discharge pipe and you get water hammer loud enough to crack cast iron flanges.
Key Components
- Double-acting piston: A solid or cup-leather piston driving fluid from both faces. Diametral clearance to the cylinder bore should sit at 0.05 to 0.10 mm for metal-on-metal builds, or zero clearance with a hydraulic cup leather. Worn pistons drop volumetric efficiency below 80% which is the field rejection threshold.
- Suction valves (×2): One at each cylinder end, opening inward when chamber pressure drops below atmospheric (or below supply head). These set the suction lift limit — practical lift is around 7 m at sea level before vapour cavitation starts pitting the valve seats.
- Delivery valves (×2): One at each cylinder end, opening outward against discharge pressure. Spring-loaded clacks are standard above 10 bar. Valve mass and spring rate must be tuned to close before the piston reverses, otherwise back-flow drops capacity by 15 to 30%.
- Piston rod and stuffing box: Rod surface finish needs Ra ≤ 0.4 µm — anything rougher chews through square-braided graphite packing in under 200 hours. The stuffing box typically holds 4 to 6 rings of packing with a lantern ring for flush water on dirty service.
- Air vessel: A pressure-tight chamber on the discharge side, sized at roughly 6 to 9 times the per-stroke displacement volume. The trapped air cushion smooths the pulsating delivery into something close to constant flow at the outlet flange.
- Crosshead and connecting rod: Converts the rotating crank motion to pure linear motion at the piston rod. The crosshead takes all the side load that would otherwise score the cylinder bore — bore wear above 0.3 mm taper end-to-end is almost always a sign of crosshead slipper clearance gone past 0.5 mm.
Real-World Applications of the Force Pump (form 2)
Double-acting force pumps in the Form 2 layout show up wherever you need steady moderate-pressure delivery from a slow-running prime mover — typically 30 to 200 RPM crank speed. The double-acting principle is what makes them viable on slow steam engines, hand levers, and donkey engines where a single-acting pump would deliver in jerks the system can't tolerate. Fire service, ship's bilge and ballast, brewery wort transfer, and heritage waterworks are the four sectors where you still see live working examples today.
- Marine auxiliaries: Worthington Simpson duplex feed pumps on coastal steam vessels, delivering boiler feedwater at 12 to 18 bar against a hot well at 80 to 90 °C.
- Heritage waterworks: The Tangye double-acting force pump preserved at Coldharbour Mill in Devon, lifting water from a leat-fed sump for demonstration runs.
- Fire service (historic): Merryweather hand-lever fire engines using twin double-acting cylinders feeding a single air vessel — the layout that threw a recognisable jet at the Crystal Palace fire trials.
- Brewing and distilling: Wort and weak-wort transfer pumps in traditional Scottish distilleries where positive displacement is required to handle viscous unfermented liquor without shearing the protein.
- Mining dewatering (heritage): Shaft sump pumps on display at the National Coal Mining Museum, recirculating water through a closed loop to demonstrate Cornish-style colliery drainage.
- Agricultural water supply: Wind-driven double-acting force pumps on remote stock-watering installations in the Australian outback, feeding header tanks from boreholes 20 to 40 m deep.
The Formula Behind the Force Pump (form 2)
The figure that matters for a Form 2 force pump is theoretical discharge — the volume delivered per minute at a given crank speed. Because both faces of the piston discharge, you get two displacement volumes per revolution, but the rod-side volume is reduced by the rod's own cross-section. At the low end of the typical operating range, around 30 RPM, the pump moves slowly enough that valve dynamics don't matter and volumetric efficiency stays above 95%. At the nominal sweet spot — usually 60 to 90 RPM for heritage iron, 120 to 150 RPM for modern bronze builds — you get peak smoothness from the air vessel and clean valve closure. Push past the high end of around 200 RPM and valve slip kicks in: the delivery clacks can't close fast enough between strokes and real output drops well below the calculated figure.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Q | Volumetric discharge | m³/min | gal/min |
| N | Crank speed | rev/min | rev/min |
| Ap | Piston cross-sectional area | m² | in² |
| Ar | Piston rod cross-sectional area | m² | in² |
| L | Piston stroke length | m | in |
| ηv | Volumetric efficiency (typically 0.85 to 0.95) | dimensionless | dimensionless |
Worked Example: Force Pump (form 2) in a restored brewery wort transfer pump
You are recommissioning a restored 1920s Tangye double-acting force pump for wort transfer duty at a working heritage brewery in Burton-upon-Trent. The pump must move 60 °C wort from a copper to a hop-back roughly 4 m above. The pump has a 100 mm bore, a 25 mm piston rod, and a 200 mm stroke. You need to know what discharge to expect at the low, nominal, and high crank speeds the leather-belt drive can deliver.
Given
- Bore = 100 mm
- Rod diameter = 25 mm
- Stroke L = 200 mm
- ηv = 0.92 —
- N range = 30 to 150 RPM
Solution
Step 1 — calculate the piston and rod areas in m²:
Step 2 — combined displacement per revolution (front face plus rod-side face), with stroke L = 0.200 m:
Step 3 — at the nominal working speed of 90 RPM with ηv = 0.92:
That's the sweet spot — about 4.2 L/s, fast enough to transfer a 30 hL copper into the hop-back in roughly 12 minutes without thermally shocking the wort.
Step 4 — at the low end of the operating range, 30 RPM:
Volumetric efficiency creeps up to about 0.95 at 30 RPM because valves have plenty of time to close. The transfer takes 35 minutes — slow but absolutely safe on the suction side, no risk of cavitation pitting the bronze valve seats.
Step 5 — at the high end, 150 RPM, ηv drops because of valve slip:
Theoretical Q at 100% efficiency would be 457 L/min, but in practice the delivery clacks can't fully close between strokes at this speed and you lose around 18% to back-flow. You'll also hear the air vessel start to ring — the pulse frequency is high enough to excite the discharge pipe at its natural frequency.
Result
At nominal 90 RPM the pump delivers 252 L/min of wort against a 4 m static head — comfortable margin for the duty and the air vessel smooths the discharge to what feels like a steady flow at the hop-back inlet. Across the operating range you cover 87 L/min at 30 RPM up to 374 L/min at 150 RPM, with the practical sweet spot sitting between 75 and 100 RPM where valve dynamics are clean and the leather drive belt isn't slipping. If you measure 200 L/min instead of the predicted 252, the three usual suspects are: piston cup-leather worn past 0.3 mm radial gap (drops ηv by 8 to 12%), a delivery valve spring fatigued so the clack lifts above its 6 mm limit and slams back late, or stuffing box packing glazed and leaking suction-side air during the back-chamber stroke. Pull the back-end cover first — it's quickest to inspect and accounts for over half the field cases we see.
When to Use a Force Pump (form 2) and When Not To
The Form 2 double-acting force pump sits between the simpler Form 1 single-acting force pump and the smoother but more complex triplex plunger pump. Pick on the basis of flow continuity, rebuild cost, and what prime mover you've got driving it.
| Property | Force Pump Form 2 (double-acting) | Force Pump Form 1 (single-acting) | Triplex plunger pump |
|---|---|---|---|
| Discharge pulses per revolution | 2 | 1 | 3 (and overlapping) |
| Typical crank speed range | 30 to 200 RPM | 20 to 150 RPM | 150 to 600 RPM |
| Practical pressure ceiling | 50 bar | 30 bar | 700 bar |
| Volumetric efficiency at nominal speed | 88 to 94% | 90 to 95% | 94 to 97% |
| Number of valves to maintain | 4 (2 suction, 2 delivery) | 2 | 6 |
| Stuffing box / packing failure points | 1 (rod gland) | 0 (no rod-side chamber) | 3 |
| Relative rebuild cost | Medium | Low | High |
| Best application fit | Slow-driven medium-pressure transfer (heritage, marine, brewing) | Hand pumps, low-duty water supply | High-pressure cleaning, oilfield, hydrostatic test |
Frequently Asked Questions About Force Pump (form 2)
The back-chamber displacement is genuinely smaller than the front by the volume of the piston rod — that's not a fault, it's the geometry. For a 100 mm bore with a 25 mm rod the back-side volume is about 6.25% less per stroke, and you'll hear it as a faint two-beat rhythm on the discharge pipe even when everything is healthy.
If the difference is more than around 10%, suspect air leakage past the rod gland during the back-side suction stroke. The back chamber pulls suction past the same packing the rod is sliding through, so any air ingress hits that side first. Smear a film of oil round the gland nut while the pump is running — if the back-stroke output picks up, your packing needs renewing.
No — and this is one of the genuine differences from a centrifugal. The cup leather or piston seal in a reciprocating pump relies on the pumped fluid for lubrication and for cooling the rubbing face against the cylinder wall. Run a leather-cup pump dry for even 30 seconds at 90 RPM and the leather glazes, then cracks. We've seen brand-new cups destroyed in under a minute on a careless commissioning fill.
If your system has any chance of running dry on start-up, fit a foot valve on the suction and a priming cock on the front cylinder cover. Crack the cock, pour water until it weeps clean, then close and start.
The decision usually comes down to crank speed and the prime mover. If you're driving from a slow-running source — a steam engine, a wind pump, a hand lever, or a heavily geared-down electric motor below 200 RPM — the Form 2 wins because it gets you two delivery pulses per rev and an air vessel can smooth the result. A triplex needs at least 150 RPM to behave well and really wants 300+ RPM, which means a fast electric drive.
If you're already running a 1450 RPM induction motor through a belt, a triplex is smoother, smaller, and cheaper per litre delivered. We'd only specify a Form 2 in that scenario for heritage authenticity or if the fluid is too dirty for plunger packings to survive.
That's roughly twice the swing you should see. A properly charged air vessel on a Form 2 should hold pulsation to ±1 to ±1.5 bar at nominal speed. The most common cause is the air cushion has dissolved into the pumped fluid — water absorbs roughly 2% of its volume in air per day at 10 bar, so a vessel that was charged six months ago is now mostly water.
Open the snifting valve at the top of the vessel, drain it down to roughly two-thirds full of air, then re-pressurise. Do this every three to six months on continuous service. If the swing comes back within a week, you've got a leaking air-vessel gasket — pressure-test it cold to confirm.
Static lift isn't the whole story — what matters is NPSH available at the suction valve, which the acceleration head subtracts from. A reciprocating pump has to accelerate the entire suction column from zero to peak velocity twice per revolution, and that acceleration head can easily reach 3 to 5 m on a long suction pipe at 90 RPM. Add it to your 4 m static lift and you're knocking on the vapour pressure of the fluid.
Two fixes work: shorten the suction pipe, or add a suction-side air vessel close to the pump inlet. The suction air vessel is the one most heritage rebuilders forget to fit, and it transforms cavitation behaviour on long suction runs.
You can — up to a point — but the returns drop off quickly above the design speed. Inertia of the reciprocating mass scales with the square of crank speed, so doubling RPM quadruples the peak rod load. Valve closure time is roughly fixed (governed by spring rate and valve mass), so as you speed up the valves close later in the stroke and volumetric efficiency falls.
The rule of thumb is: if you need 50% more capacity, run faster. If you need double, fit a bigger cylinder. Above about 150% of design speed on a heritage Form 2, you'll start cracking valve seats within months.
One-sided rod scoring is almost always misalignment between the crosshead guide and the cylinder centreline. A Form 2 pump has the crosshead carrying all the lateral component of connecting-rod thrust, and if the crosshead slipper clearance is uneven or the guide is set off-axis by even 0.2 mm, the rod gets pushed sideways through the gland and chews packing on one side.
Pull the rod, blue the gland bore, and rotate the rod through it by hand — you should see contact all the way round. If contact is concentrated on one arc, re-shim the crosshead guide before you fit new packing or you'll destroy it again in another 300 hours.
References & Further Reading
- Wikipedia contributors. Force pump. Wikipedia
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