Sulphuric acid at 98% concentration is one of the most demanding fluids you will ever ask an AODD pump to handle. It is not just corrosive — it is dense, with a specific gravity of 1.84, nearly double that of water. It generates heat on contact with moisture. And it will destroy the wrong pump material within days.
Yet it is pumped routinely in chemical manufacturing, water treatment, and industrial processing. The challenge is not whether an AODD pump can handle it — the right configuration can. The challenge is understanding exactly what makes this fluid so unforgiving, and what you need to get right to avoid constant diaphragm failures.
Why Sulphuric Acid 98% Is Different
Most operators focus on chemical compatibility when specifying a pump for sulphuric acid. That is the right instinct, but it is only part of the picture.
The specific gravity of 1.84 means that every moving part of the pump — the diaphragm, the check valves, the air distribution system — is working against a fluid that is nearly twice as heavy as water on every single stroke. At the same pump speed and air pressure, the mechanical load on the diaphragm is dramatically higher than it would be with a lighter chemical.
This has two practical consequences that operators frequently underestimate. First, PTFE diaphragms — which are required for chemical compatibility with 98% sulphuric acid — are already the most mechanically rigid option available. They do not flex and recover like elastomers. They cold creep under sustained load, taking a permanent set over time. High specific gravity accelerates this process significantly.
Second, the static head pressure generated by a tall tank of sulphuric acid is far higher than the same tank filled with water. A 10 metre column of 98% H₂SO₄ generates approximately 1.80 bar of static head at the pump inlet — nearly double what you would get from water at the same height. If this is not accounted for, the diaphragm is fighting that pressure on every stroke before the air supply even engages.
The Most Common Failure Pattern
The failure we see most often in sulphuric acid applications is PTFE diaphragm deformation leading to shaft hole tearing, typically within two to six weeks of commissioning.
The cause is almost always a combination of factors rather than a single mistake. High static inlet pressure from a tall tank pushes the diaphragm backward on the return stroke beyond its designed travel. The operator, seeing that flow is lower than expected, increases the air pressure to compensate. Higher air pressure increases the mechanical load per stroke. The pump is also running faster than it needs to because the air pressure is too high for the system resistance. More cycles per hour on an already overstressed diaphragm — the failure accelerates rapidly.
By the time the customer calls, the diaphragm has been accumulating cold creep damage since day one. The pump did not suddenly fail. It failed progressively, and the installation conditions were the cause.
What the Correct Installation Looks Like
Getting sulphuric acid applications right requires attention to four things simultaneously.
Material selection. The pump body must be PVDF — polypropylene will not survive extended contact with 98% sulphuric acid at any meaningful concentration. The diaphragm must be PTFE. The balls and ball seats should also be PTFE for the widest chemical resistance. There is no compromise on this combination for this fluid.
Inlet pressure management. Calculate the static head pressure at the pump inlet before commissioning. Use the actual fluid density — SG 1.84 — not water. If the pump is installed below a tall tank, this pressure can be significant. Reduce the air supply pressure to account for it. The diaphragm faces pressure from both sides simultaneously, and the combined load must stay within the pump’s designed operating range.
Air pressure discipline. Set the air supply to the minimum needed to achieve the required flow and head. Do not set it to the maximum available and assume the pump will handle it. With sulphuric acid at SG 1.84, every unnecessary bar of air pressure translates directly into additional diaphragm stress. Install a precision regulator with a gauge so the actual setting is visible and controllable.
Stroke rate control. Install a needle valve on the air exhaust to control stroke speed independently of pressure. Slowing the stroke rate reduces flex cycles per hour and extends diaphragm life significantly — particularly important with PTFE, where accumulated flex cycles directly determine service life.
Check Valve Considerations
One detail that is frequently overlooked in high specific gravity applications is check valve performance.
AODD pump check valves are typically designed and tested for water. At SG 1.84, the buoyancy force on the ball is substantially higher than in water, which can reduce the ball’s ability to seat reliably between strokes. The result is incomplete valve closure, backflow of fluid, and erratic pump behaviour that operators often mistake for a diaphragm or air valve problem.
For sulphuric acid applications, confirm with the pump manufacturer that the check valve configuration is appropriate for the fluid density. In some cases, a heavier ball material or modified seat geometry is recommended.
Suction Line Design
The suction line for a sulphuric acid application deserves the same care as the pump itself.
Keep the suction line short and direct. Any unnecessary length adds friction loss that the pump must overcome on every stroke. Size the suction pipe at least equal to or larger than the pump inlet — never reduce to the pump connection size with an abrupt transition close to the inlet. An abrupt pipe reduction near the pump inlet creates hydraulic surges that impose additional shock loading on the diaphragm with every stroke cycle.
If the pump is drawing from a drum or IBC, ensure the suction line and foot valve are correctly sized for the fluid viscosity and density. Sulphuric acid 98% has a viscosity slightly higher than water at room temperature, but this increases as temperature drops. In cooler environments, check that suction conditions remain adequate throughout the operating range.
Inspection Intervals
Standard diaphragm inspection intervals are not appropriate for sulphuric acid service.
The mechanical demands of SG 1.84 combined with the cold creep characteristics of PTFE mean that diaphragm wear accumulates faster than in lighter fluid applications. Shorten your inspection cycle accordingly. Do not wait for the diaphragm to fail before inspecting it — by that point, the acid has already entered the air side of the pump and potentially caused secondary damage to the air valve and centre block.
A failed PTFE diaphragm in sulphuric acid service is not just a consumable replacement. It is a safety incident and a process downtime event. The cost of a proactive inspection schedule is small compared to the cost of an unplanned failure.
The Bottom Line
Sulphuric acid 98% is a manageable fluid for an AODD pump when the installation is designed correctly. The right material specification, controlled air pressure, disciplined stroke rate, and adequate suction design will give you reliable service life from a PVDF/PTFE configuration.
Ignore any one of these factors, and the specific gravity alone will shorten your diaphragm life to a fraction of what it should be — regardless of the pump brand or the quality of the diaphragm material.
Dealing with frequent diaphragm failures in a sulphuric acid application? Contact the Autoflo team. We can review your installation conditions and help you get it right.