Every once in a while I have to confront an opinion that Multi-Function valves from Danfoss series 90 closed-loop pumps are "all the same". There is a big chance that this myth is a local Portuguese phenomenon that emerged due to the lack of technical curiosity on the part of certain dealers, who traditionally give more attention to the commercial side than to the technical details of the product they sell and promote. In this article, I will explain, yet again, the function and principle of operation of these valves, as well as the difference in the dynamic behavior between their types.
First, let us review the basic operation. The multi-function valve of a Danfoss series 90 closed-loop pump performs four functions:
The check valve function allows the charge oil to enter the low-pressure side of the loop. The function is performed by the two-piece poppet located in the nose. (cutaway view). The outer ring lifts the smaller poppet from its seat and lets the charge flow fill the loop. This assembly functions as a logic element check valve configured for annulus flow (free flow from side to bottom).
The by-pass function inter-connects the loop legs when necessary (for emergency towing, for example), and is activated by loosening the middle hexagonal actuator (which, by the way, has a very "thin-ish" design, in my opinion. I'd make the hex at least 5 mm thicker). This loosens the pre-load of the pressure spring and thus turns the assembly into an open relief valve, which will return to its original setting when the hex is re-tightened. Not a by-pass by-pass, per se, but it does the job, I guess.
The pressure limiter function reduces the pump's displacement when the high-pressure setting is reached. The function is performed by the tiny poppet, pre-loaded by the adjustable spring (cutaway view). When the system pressure lifts it from its seat, the oil flows directly into the servo-cylinder opposite to the one that is tilting the swashplate, which reduces the effective delta-P of the servo cylinder and de-strokes the pump.
And finally - the pressure relief function, which cuts through the fast pressure surges by venting the high pressure into the low-pressure leg of the loop when the limiter function isn't fast enough. The inner part of the two-piece poppet (the outer part provides the check valve function, the assembly would work with a single-piece poppet, but the two-piece design makes the relief poppet much lighter and therefore increases its opening speed) acts as a logic element piloted by the pressure limiter poppet.
It is the latter two functions - the limiter and the cross-port relief - that have different behavior according to the type of a multifunction valve. The visible difference between the three types that you see in the pictures below is the diameter of the orifice in the nose poppet, and, obviously, the numbers stamped on the side:
The size of the orifice defines the dynamic behavior of the logic element. Let's look into it. The relief poppet moves when the force, created by the high pressure, overcomes the force of the spring. When the poppet lifts from its seat and the oil is directed to the servo-cylinder, the flow through the nose orifice starts to increase, and so does the pressure differential acting across the nose poppet (the logic element). However, with a smaller orifice, it takes less flow to reach the delta P sufficient to lift the nose poppet than it is with a bigger orifice. At the same time, the orifice is a flow-limiting factor, which conditions the flow to the servo-piston and therefore the destroking speed. Thus, by reducing the size of the logic element orifice we make the cross-port relief function faster and the desroking limiter function slower.
The official catalog states that the relief function of a multi-function valve is sequenced to operate at approximately 35 bar above the pressure limiter setting and that the relief response time is approximately 20 ms. The keyword is "approximately". The actual values will depend on how fast a pressure surge is, which will be determined by the system design and operating conditions. However, if we tested the three types (type 1, 5, and 2) of the multi-function valves under the same surge rate, we would obtain different results:
Under repeating conditions, the type 2 valve would spend more time cross-porting, thus causing the oil to heat more, but also keeping the pressure surges at a lower level if we compared it with the type 1, for example, which would provide a faster limiter function, at the expense of higher pressure spikes.
The correct choice of a multi-function valve is determined by the system it is used in, so special care should be taken when replacing damaged valves. By saying "special care" I do not mean that it is imperative to replace a broken multi-function valve with the same type. What I mean is - common sense - i.e realizing what will alter if the valve type is changed, and then seeing if it's acceptable.
The reason I am putting only the types 1, 2, and 5 here is simple - these three are the only ones I've seen pass through our shop. According to the Danfoss parts list for series 90 there are more.
The multi-function valves for series 90 have a simple and compact design, and as a rule last as long as the pump itself. I have seen them shatter and cause a catastrophic pump failure, but it's not an everyday malfunction. If I could change one thing about these valves - it would definitely be the hexagonal adjusters! I would make them taller and therefore easier to clamp a standard wrench on.