Rats and the damn dirty nests they live in—that’s why you should care about manifolds. Nobody likes a rat’s nest. Well, perhaps little baby rats care about a rat’s nest. And mommy rats. Mommy rats adore rats’ nests. Okay, so probably all rats like a good rat’s nests. Any why not? They’re warm and snug, and make for a great afternoon siesta. But you and I? We want nothing to do with a rat’s nest.

A fluid power rat’s nest is a shamble of connectors, fittings, adaptors, tubes and hoses. It’s when your pneumatic circuit looks like your wadded white iPhone headphones when you put them in your pocket for a third of a second. It’s like when your hydraulic plumbing looks like L.A.’s Four Level interchange. We’ve all seen those jury-rigged systems where they’re trying to split off ports, connect ORB to metric or join multiple return lines together. It’s not pretty, folks.

In simplest terms, a manifold is a component from which you attach other things. That’s probably too simple an explanation, because it also describes a wall, a Lego block and a telephone pole. A slightly less elementary explanation is that it cleans up plumbing—and this is why you should care.

A manifold can be used to split a primary source of fluid off into secondary circuits, or conversely, be used to join together exhausted fluids. A manifold can also be used as a plumbing standard for which to attach valves. In some cases, a manifold can be used to replace fluid connections in an entire circuit. I’ll elaborate on these three primary uses, all of which are pragmatic options for engineers, technicians or millwrights to exploit.
For plumbing purposes, a manifold is excellent for cleaning up a rat’s nest. There are many reasons that a rat’s nest exists. It could be poorly thought out installation of a new system, where the technician was given a circuit to plumb, but no other resources but a shelf of salmagundi fittings dating back to the Reagan administration. The nest could have resulted in years of upgrades and modifications to an old machine, where technicians were too lazy or unresourceful to plumb it correctly.

Don’t get me wrong, I’ve seen some clean installations without the use of manifolds, but those installs used proper tubing, clamps and fittings all precisely located with premeditated drawings. However, even these kinds of machinery can benefit from headers and manifolds to clean up plumbing.

To be qualified as a manifold, it will need to contain at least three ports to connect tubes or hoses–otherwise, it’s just an adapter. A tee adapter has three ports, but unless it has all female ports, it’s disqualified from being a manifold. Also, fittings are typically cast, molded or forged to their shape. A manifold can start as a cast or forged bit of aluminum, ductile iron, steel or even plastic, but it is then typically machined and drilled for ports.
A header is a manifold with ports machined down its length, and traditionally has a larger port at one or both ends. You might see SAE 12 ports at each end and SAE 8 along its length. The port locations along the length could number from two to ten or more. Also, you can have dual side ports at each location, either directly opposing each other or at 90°. You could also have ports at all four sides of the manifold, although you can risk reducing the strength of the block.

There also exists a junction block, which joins two or more ports at each location of the manifold only, and lacks the gun drilling down the length of the block. Rather than join all ports together, it joins just the ports at each section. This is a great way to clean up plumbing at locations where conduits change direction, interface or require maintenance or disconnection frequently. They could also have quick couplers plumbed directly to each port, a nifty move I’ve seen on injection molding machines, for example. This allows for quick changeover of molds, as lines are quickly removed and reattached.

Aluminum and ductile iron are the two most common metals I’ve seen used for manifolds. They are easy to machine, relatively inexpensive and ready available. Aluminum manifolds are versatile, and can be used in both pneumatic and medium pressure hydraulic applications. Although aluminum isn’t typical for use in high-pressure hydraulic applications, you can beef up the side wall thickness to allow 3,000 psi operation, especially with better alloys, such as 6061. Aluminum is the top choice for integrated circuits, which I’ll describe later.

Ductile iron is an cast alloy with graphite, and is less brittle than cast iron. It machines well, and is strong enough to handle 5,000 psi or more in manifold applications. It is quite heavy, so is sometimes used sparingly, especially for mobile applications sensitive to unnecessary mass. For high-pressure industrial applications, it’s the standard because mass of stationary machinery is usually a non-factor.

The header is typically used after a pump to split off plumbing lines into sub-circuits. You might see it after a pressure-compensated pump, where multiple lines are distributed to individual closed center valves at locations across the machine. A header is far superior in form and function to a similar setup using fittings alone. Not only does it look cleaner, it reduces the opportunities for leaks, both at machine start-up and after periods of maintenance.

Even more important, in my opinion, than a pressure header is a tank line header. It seems there is much more thought put into the plumbing of pressure lines than tank lines, and I imagine more than one machine heard the words, “Okay, now what do we do?” If you do one thing to clean up your plumbing, please use one or more tank line manifolds. Once your tank lines are a mess, it gives all future technicians working on the machine no motivation to keep the plumbing clean, and it goes downhill from there.

The second common use for manifolds is as subplates for valves. There are a few common valve standards, the most common being the industrial CETOP/ISO/NFPA valves, which are commonly referred to by their sizing of D03, D05, et al. These valves are usually solenoid operated, with anywhere from four to seven ports in the bottom, where it is interfaced with the machined manifold. They attach the valves to the manifold with cap screws.

If the manifold is designed to contain just one, it’s called a subplate. It will be machined with the previously mentioned drillings to interface with the valve, but those drillings will terminate with port cavities for pressure, tank and work lines. Most often, the subplates will contain just those four ports, plus through holes for mounting the manifold to a rigid surface. Subplates are absolutely required for all ISO valves, because if the valve has work ports already, it is a different standard entirely.

Sometimes the valve manifolds are manufactured to accept multiple valves, combining the benefits of a subplate and header. The manifold contains all the pressure, tank and work ports to create a complete hydraulic circuit, and is a tidy method to plumb up a hydraulic circuit, especially since the manifold often contains its own cavity for a relief valve. Using the versatility of ISO valves, this means a manifold can have all the pressure, flow and directional valves attached to itself, creating a complete hydraulic circuit.

However, there is one downside to the stacks of valves piled atop a manifold; the opportunity for leaks. Generally, a valve stack doesn’t leak until years of heat cycles lead to brittle O-rings, but it does happen. To sidestep this issue, we can take advantage of the third common manifold use, which is the integrated circuit.
The integrated circuit uses a single block of machined metal, and no matter the complexity of the circuit or number of valves, the only place for external leakage is the connection ports. Cartridge valves are spools or poppets built from a single axially aligned cartridge, with O-rings separating each cavity. The valves are threaded and screwed into the block, which itself is a series of passages and cross drillings, which can get quite complex.

Because of the myriad cartridge valves available, such as from Sun Hydraulics or HydraForce, a hydraulic circuit can be as complex and specific enough as you can dream up. I’ve personally designed circuits with more than seventy valves, including load sense check valves, compensators, directional valves, proportional flow controls, and so forth. However, the same block with seventy valves still only had a dozen ports, so you can imagine how much this cleans up plumbing. Plumbing seventy separate valves would warrant intervention from FEMA.

Sometimes, the performance of cartridge valves can be limited, as their compact nature makes the integrated circuit often flow very little. As flow increases in a system, the cartridge valve options decrease, especially greater than 40 gpm or so. Going much higher in flow above this range requires logic elements in the form of large poppet valves operated by pilot valves. All the same trick valves can be used in controlling these valves, but it adds complexity and size quickly.

Extremely high flow systems use slip-in cartridge valves, and in fact, they’re the only way it makes sense to flow ridiculous volumes of fluid; we’re talking hundreds of gallons per minute. I’ve seen hydraulic manifolds as big as a Smart car, being fed by four pumps and containing over a hundred valves, including ISO valves mounted directly to the block. If you think a rat’s nest looks bad for a 10 gpm system, imagine trying to plumb 700 gpm worth of hoses, tubes and fittings in close proximity.

If you’re a designer and you want your machine to be taken seriously, especially by those in the industry, you must absolutely use manifolds wherever possible. Their ability to clean up a hydraulic or pneumatic circuit cannot be overestimated. Even if you care less about future maintenance, you should care about appearance. You’re not a rat, after all.