Most pneumatic conveying systems are designed from the blower backwards. The pipe diameter is calculated, the bends are counted, the filter is sized, and the valves are fitted into whatever space is left. Then the system runs at 20 percent more air than the calculation promised, and the valve becomes the suspect. This note is about where that pressure actually goes, what each valve type adds, and how to compare two options before you buy rather than after you commission.

A valve is a small item on a long line, and on a dilute phase system it really is a small part of the total pressure budget. The problem is that it is the part with the least predictable geometry. Pipe is pipe: a fabricator knows the bore and the bend radius. A valve has a body, a seat, an actuator, an obstruction in the flow path and, on a diverter, a branch that changes the direction of the material. All of that sits in the line and none of it appears in the pneumatic conveying calculation unless someone puts it there.

The result is a system where the blower is chosen for a pressure that the line does not actually deliver. The usual quick fix is to raise the air volume, which raises the velocity, which raises the wear on every bend downstream of the valve. That is how a pressure drop problem turns into a maintenance problem eighteen months later.

Pressure drop only means something against a baseline, and the baseline is set by the conveying mode.

High velocity, low solids loading, and a system pressure usually below 1 bar. The material travels in suspension at anywhere from 15 to 30 m/s, and the pressure falls fairly evenly along the line because friction is distributed. In this mode a valve might account for a few percent of the total, but because the whole system sits at low pressure, a few percent is the difference between a standard blower and the next size up.

Low velocity, high solids loading, and system pressure from 1 to 6 bar. Material moves as plugs or waves at 3 to 8 m/s. The pressure profile is lumpy: it climbs while a plug forms, holds while the plug travels, and drops when it clears. The valve's share of the total is smaller in percentage terms — the absolute numbers are simply larger — but the valve now sits in a high pressure region and has to hold that pressure while it operates, which is a mechanical requirement rather than a flow one.

If you are still deciding which mode suits your material, the general comparison is on our pneumatic conveying valves page; the rest of this note assumes you have picked one.

The figures below are the order of magnitude we see in practice, expressed as equivalent length of straight pipe of the same bore. They exist to let you compare types against each other. They are not a substitute for checking your own line, and they assume a fully open valve.

Two things follow from that table. The first is that a full bore pinch valve is not merely low loss, it is close to invisible to the system, which is why it is common on abrasive duty where you cannot afford velocity. The second is that on a diverter the pressure loss belongs to the branch geometry rather than to the valve, so moving the branch or lengthening the transition can matter more than changing the valve.

Our VF series air-operated pinch valve is the full bore case: DN40 to DN300, flanged aluminium body, the sleeve doing the sealing and the bore staying continuous. For line switching the choice between KFQ ball diverter, KFZ plug diverter and VAB flap diverter is usually made on sealing duty and material abrasion rather than on pressure drop alone. Where a compact shut-off is needed between flanges, the PZD inflatable seated butterfly valve trades a small permanent loss for a very short face-to-face length.

Air is the expensive part of a pneumatic system, and it is expensive in proportion to pressure. A blower or compressor delivering a given volume against a higher pressure draws more power, and the relationship is close to linear within a given machine's range. Raise the line pressure drop by 10 percent with the volume unchanged and you pay roughly 10 percent more to run the air supply, every hour, for the life of the plant.

That is the honest argument for a low loss valve. It is rarely won on purchase price, because a full bore valve usually costs more than a wafer butterfly. It is won on the running cost, and on the fact that a valve that does not add velocity does not add wear to the bends behind it.

Dense phase changes the arithmetic. There the compressor is working at several bar, and compressed air at that pressure costs considerably more per cubic metre than low pressure blower air. In dense phase the case for reducing any restriction is stronger, not weaker.

Position matters as much as type, and it is the part most often ignored because the layout is fixed by the plant rather than by the conveying engineer.

A valve's pressure drop is not fixed for life. Two things move it.

The first is seat wear. An elastomer seat that has taken abrasive material loses its profile, the sealing line widens, and a valve that sealed at the start of the campaign starts leaking internally. On a diverter this is worse than on a shut-off valve, because a leaking idle leg steals air from the conveying leg without showing up as a visible leak.

The second is sleeve or seat swelling and hardening. Natural rubber hardens with heat and with certain oils; EPDM resists both but swells in some hydrocarbon service. A hardened sleeve opens less fully at the same air pressure, which quietly throttles the line. If a system's capacity drifts down over a year, check the sleeve before blaming the blower.

That is why we treat the sleeve and the seat as scheduled wear parts rather than as failures. Material selection for those parts is covered in more detail in our powder and bulk solids valve guide.

Before specifying, put these four numbers on one line:

If the answer to the third point is no, the pressure drop argument is already lost — a reducer into a smaller valve costs more than the valve saves. That single check eliminates most of the bad selections we see.

If you send the material, the rate in tonnes per hour, the conveying distance and a sketch of the line with the valve positions marked, we will tell you which valve we would fit at each point and where we think the pressure will actually go. If the answer is that our valve is the wrong choice for a position, we will say so. Write to zjxnjx@126.com, or see the full range on our product list.

ZHEJIANG XINGNENG MACHINERY CO., LTD. manufactures pinch valves, diverter valves, iris valves and inflatable seated butterfly valves for powder, bulk solids and pneumatic conveying duty.