Most horizontal mill manufacturers will tell you they do not recommend using air diaphragm pumps with horizontal media mills because of the process problems that can occur. These process problems can include high pressure shut off, and high product temperature, accelerated parts wear, and seal failures among others. Why do these problems occur? The obvious answer is that if the “pulsating” flow of an air diaphragm pump is not properly controlled, it can mess up the balanced flow pattern required for good dispersion in a horizontal media mill. This can cause the dreaded “hydraulic packing” of grinding media to the screen end of the mill. Plus, air diaphragm pumps have high pumping capability, usually measured in GPM’s in a world where horizontal media mills usually run in GPH’s.
How do you control this excess capacity without actually making the pulsating effect worse? And why doesn’t this happen in a vertical mill? Very simply the vertical chamber acts as a natural pulsation dampener or surge suppressor. Plus you are pumping from bottom to top; you have gravity working for you.
In the real world, I know of dozens of customers who have switched to air diaphragm pumps out of necessity and frustration with pump repair bills. Many of them put up with some process problems because of all the inherent advantages of air diaphragm pumps such as:
- They are inexpensive to purchase (although the air to run them isn’t cheap)
- They can run dry without damage They can run “dead-headed” without damage
- They can tolerate getting filled with “grinding media” with little or no damage
- They are “self-priming”
- They easily pump “thixotropic” or shear-thinning materials without the “internal slip” common to rotary gear or lobe pumps
- They easily pump cleaning solvents through the mill at high flow rates and linear velocity to help cleaning, while still being able to handle more viscous mill bases at normal process flow rates
How can I use an air diaphragm pump with my horizontal media mill without causing process problems? Find an effective and inexpensive way to smooth out and control the flow to the horizontal milling chamber. One way is with a “surge suppressor” or “pulsation dampener” that many manufacturers offer. These in-line devices consist of a chamber with diaphragm which operates with an air cushion and works counter to the stroke of the pump. These can be effective in some applications but are not in others and create another maintenance issue, and they can cost almost as much as the pump itself.
Another way to minimize pulsation and control flow is to install a dual valve by-pass loop in your pipeline connecting the pump to the process (see attached conceptual sketch). A by-pass line with a valve is connected form pump discharge piping back to the pump suction piping. The valve is used to regulate flow through the by-pass line. Downstream in the pump discharge line, another valve is installed to serve as the flow control valve to meter flow to the horizontal milling chamber, or any other device for that matter. Gate style valves are recommended because they are easier to control than a ball valve, for example. It takes more turns to change the flow through a gate valve which means better control. Any type of “slow acting” valve is acceptable here.
This “dual valve” by-pass loop provides ultimate control of an air diaphragm pump. Another benefit is that it allows the air diaphragm pump to operate at a higher frequency of strokes. When metering downstream flow from a “higher frequency” supply stream, the resulting metered flow is significantly smoother. The pulsation has been minimized-without use of a surge suppressor.
This concept is much more effective than trying to throttle the air supply to the diaphragm pump. How reliable and repeatable is a process in which you are trying to control a pump that is chugging only a few times a minute? Even with improved new air valve mechanisms, the problem of “stalling” or clogging of the feed lines may occur. By the way, do all of us have brand new pumps? Naturally, this dual valve by-pass loop is not suitable for all applications. Cleaning can still be a problem, but you easily speed up an air pump to get the higher velocity flow you need to clean the added piping. All in all, there are many practical everyday reasons to consider this approach.