When the desired vacuum condition isn’t provided at a process plant, production often comes to a halt and all eyes become focused on the vacuum pump as the root cause of the problem. However, the vacuum pump usually isn’t culprit. In almost all cases, either: 1) the pump is being operated in a condition for which it never was intended, 2) one or more of the user’s interface points with the pump (suction/discharge lines, water supply, process contaminant, etc.) are being operated outside of design parameters, or 3) the vacuum chamber or vacuum lines were improperly specified. Each vacuum pumping technology will react differently to various conditions, so it’s not possible to offer a “one size fits all” answer to the problem. The following is a guide to systematically identifying the root cause of the most common problems and correcting them based on general vacuum system recommendations as well as technology-specific issues.
Let’s start by noting that vacuum technologies found at plants generally fall into two categories: wet and dry. The terms “wet” and “dry” refer to whether or not the user’s process gas comes into contact with a liquid as the gas passes through the vacuum pump. Wet technologies utilize a liquid to create a seal between the discharge and the suction of the pump to minimize the “slip” of gas backwards from the discharge to the suction and increase volumetric pumping efficiency. Dry technologies have no liquid contact with the process gas. Table 1 lists common vacuum equipment of both types.
The following points apply to all vacuum systems regardless of pump type:
Vacuum leaks. All vacuum systems have some amount of air-in leakage, which may or may not be known at the time the vacuum pump is sized. Excessive system leaks result in reduced process gas pumping capacity because the pump must move not only the process gas from the vacuum chamber but also the air-in leakage. Leaks occur at the joints of the vacuum lines and at the vacuum chamber. To avoid excessive air-in leakage, bear in mind the general recommendations of operating pressure ranges for various piping materials and joining methods detailed in Table 2. Note that actual limits will depend upon the skill level of assembly personnel.
Vacuum pump or system problem? You must determine if the issue is caused by the pump or by other equipment in the vacuum system. To find out, mount an isolation valve and an accurate vacuum gauge in-line as near to the suction connection of the vacuum pump as possible. Close the isolation valve and then measure the ultimate vacuum (also called blank-off) performance of the pump. Compare the measured vacuum to the manufacturer’s published ultimate vacuum value. A value reasonably close to the published one indicates the issue stems from leaks or outgassing in the vacuum system.
Excessive pump discharge or backpressure. A vacuum pump is designed to discharge to atmospheric pressure or just slightly above unless the manufacturer specifically designates it a compressor. As the discharge pressure of the pump increases above atmospheric pressure, this raises the differential pressure across the pump, resulting in:
• higher pump temperature and possible overheating, leading to pump seizure; and
• increased current draw and subsequent overheating of the electric motor or an overload/fuse/breaker fault.
Improperly sized suction and discharge lines. Sizing of system piping significantly affects pump performance and should be performed by qualified vacuum engineers. However, to avoid problems, apply the following guidelines:
• Suction and discharge lines never should be smaller than the suction or discharge connection size on the vacuum pump.
• For every 50 ft of suction or discharge piping, increase the pipe size by one nominal pipe diameter. Example: A vacuum pump has a 2-in. inlet connection. The suction line between the pump and the vacuum chamber is to be 70 ft long. To avoid restrictions to gas flow and pumping performance issues, increase the vacuum line to 3 in.
Isolation of pumps operated in parallel. Many vacuum pump installations consist of multiple pumps operating in parallel and utilizing a common suction and discharge header. For these type of installations, isolate idle pumps from those in operation at the suction and discharge. Failure to isolate the offline pumps may result in: 1) discharge gas from the operating pumps entering an idle pump and contaminating it, and 2) creation of vacuum in the idle pump and a resulting liquid back-stream into the vacuum lines and chamber.
Now, let’s look at specific issues that might affect particular equipment.
Liquid Ring Pumps
Several possible operating conditions can cause insufficient vacuum in liquid ring (LR) pumps. The most common are: