Why Worry about Vibration?
Every machine vibrates as it operates. No matter how rigidly a machine is mounted, the machine and all attached structures will experience some undesirable motion caused by various forces. These forces are usually related to the movement of various parts within the machine. If this vibration-related movement becomes too great, damage to the machine will result.
What Causes Vibration?
Vibration can be caused by a variety of conditions including bent shafts, unbalance in rotating parts, worn or bent gears, damaged bearings, misaligned couplings or bearings, electromagnetic forces, etc.
In compressors, however, the most common causes are unbalance in rotating parts and abnormal aerodynamic forces. Ariel takes special care in the design and manufacturing processes to prevent these conditions from occurring.
How Vibration Affects Compressors
When you consider how vibration happens in a compressor, you have to consider the compressor unit and all peripheral equipment attached to it. The compressor unit(s), dryers, intercoolers, piping, etc., all combine to form a complex mechanical system that transmits vibration energy.
Natural Frequency - Ringing the Tuning Fork
This mechanical system forms a structure that has one or more natural frequencies. The best example of a natural frequency is striking a tuning fork. The tuning fork will emit a sound at its natural frequency. Every structure has a natural frequency. When an external force (e.g., a hammer blow) excites a structure, it will "ring" at its natural frequency until damping forces within the structure stop the vibration. Tuning forks will ring for a long time but not forever. Mechanical structures, which should be designed not to ring easily, won't ring for long - unless the exciting force is continuous.
In real machinery when a structure enters resonance (vibrates at its natural frequency), vibration amplitudes are magnified. Breakdowns will result at an accelerated rate.
Ariel compressors are designed so that the system's natural frequencies are far away from normally occurring vibration frequencies.
Types of Vibration in Compressor Systems
The two predominant vibration types in compressors are translational (lateral) and rotational (twisting) vibration. An example of translational vibration is the motion of external piping due to resonance. When the vibration of external piping becomes excessive, it is almost certainly because some vibration within the system is occurring at a frequency near the natural frequency of the piping structure. A common occurrence of rotational vibration is an extreme vibration along the crankshaft whose frequency is the same as shaft speed. This type of vibration is usually due to unbalance forces acting on the crankshaft caused by unequal weights in piston pairs.
Unbalance - The Primary Vibration Concern in Compressors
Most vibration problems in compressors are related to unbalance. A high vibration level may be directly related to a rotational force or a translational force that is caused by an unbalance condition.
What is Unbalance?
There are two basic types of unbalance. An unbalance force is a force that attempts to push the compressor and its foundation first in one direction and then pulls in the opposite direction. An unbalance couple is a torque that tries to pull the machine around an axis of rotation such as the center line of the crankshaft. Although these two unbalance types are similar in their effect, preventing them requires understanding how they are caused and prevented.
Let's take a closer look at unbalance force. This force tries to pull the compressor and base back and forth along a single plane of motion. This force is a result of unequal reciprocating weights (piston pair assembly weights) and aerodynamic forces of compression between piston pairs. Assuming that the aerodynamic forces are within the design capacity of the rod assemblies (as in normal operation), we need to examine the issue of reciprocating weight.
Reciprocating weight is defined as the weight of the piston assembly, balance nut, crosshead assembly, piston rod, and the "small end" of the piston rod. This is the portion of the compressor assembly that moves back and forth in each cylinder. When the difference between reciprocating weights of a piston pair is large, enough unbalance force occurs to start causing problems.
Ariel, as part of our ISO9001 certification, carefully tracks the reciprocating weights for all production units. A complete balance record for each new machine is retained at the factory. Our tolerances are within 1lbf for frames up to the JG/A/M/P/N/Q/R/W/J models, 2lbf for frames larger than the JGH/E/K/T/C/D and 5lbf for frames larger than the JGB/V/U/Z. For a 1000HP frame, this tolerance equates to less than a .33% difference between reciprocating weights in a piston pair, which is an extremely high precision standard. Exact tolerance figures for each Ariel compressor can be found in the unit databook.
Rotating weight is defined as the weight of the crankshaft and attached structures (oil slinger, auxiliary end drive, etc.) and the "large end" of the connecting rod. When variations take place in any one of these assemblies or castings, it will generate a force that tries to pull the entire unit around the axis of crankshaft rotation. The direction of force, or pull, will be in the direction where the "heavy spot" is as it moves around the crankshaft center line.
How Ariel Design Minimizes Unbalance
In order to understand how the Ariel design philosophy reduces unbalance problems in a compressor, we need to explain more about the forces affecting the compressor internal assemblies.
Horizontal Primary and Secondary Forces
The horizontal primary force (HPF) is the force of the reciprocating weight of a piston assembly trying to pull the crankshaft into the cylinder as the piston moves into the cylinder.
This force can be offset by the use of a counterweight that opposes the piston assembly during rotation. The horizontal secondary force (HSF), which is much smaller than the primary force, is due to the connecting rod motion around the crankshaft center line.
Vertical Primary Couple
By using a counterweight, however, a new force is introduced when the rotation is at the midpoint of piston throw. At this stage of rotation, the counterweight is thrown into a direction perpendicular to the piston motion, and all of the counterweight's mass translates into a vertical force (vertical primary couple, or VPC) pulling the machine upwards (or downwards at the other half of the cycle).
Ariel minimizes the need for counterweights by using only paired, opposed cylinder frame designs. Now the reciprocating forces are balanced by using opposed cylinders. Since there is a much smaller counterweight in use, the vertical primary couple (VPC) is reduced to negligible levels.
Since the piston pairs cannot be directly opposed and must be horizontally offset, there is some torsional force created. These forces are called the horizontal primary couple (HPC) and horizontal secondary couple (HSC). The relative force created is determined by the offset distance "D" between the center lines of the opposing throws.
Most compressor manufacturers place the counterweights directly on the crank pin webs to reduce the HPC, but this does not reduce the HSC.
Ariel places the counterweights outboard of the main bearing webs. This reduces the size of weights needed and helps reduce both HPC and HSC.
Ariel strongly believes our opposed piston design gives you the maximum mechanical advantage in system reliability and service life. Each of our 2, 4, and 6 cylinder designs presents varying degrees of force relationship. All designs are engineered for long service life, but increasing the number of cylinders has the force relationships as defined by the following chart:
|Horizontal Primary Force
|Horizontal Primary Couple
|Vertical Primary Couple
|Horizontal Secondary Force
|Horizontal Secondary Couple
Preventing Vibration-Related Failures
Although you won't be balancing an Ariel unit by changing the weight of internal components, there are some preventative things you should do. It should be obvious by now that there are many forces exerting themselves within a normally operating compressor.
Anchor Bolt Maintenance
The fasteners and castings in your Ariel unit are designed to withstand all normally occurring vibration and torque forces. What Ariel cannot control is maintenance of the mounting fasteners that keep your unit bolted down.
Ariel strongly recommends a schedule for anchor bolt maintenance. We have encountered many problematic units in the field whose only problem was a loose mount. We suggest the following schedule:
- Initial Maintenance: The anchor bolts are tightened and released three times, with final tension set on the third tightening.
- After 7 Days of Operation: While the equipment is still near operating temperature, check for proper tension - do not loosen.
- After 30 Days of Operation: While the equipment is still near operating temperature, check for proper tension.
- Every 6 months, Starting 6 Months from Installation: While the equipment is still near operating temperature, check for proper tension. Enter correcting Resonance Problems in External Structures
We stated earlier that every system has one or more natural frequencies. If an excitation occurs at one of these frequencies, a structural system will tend to ring. Existing vibration levels will be exaggerated and damage to the machinery will occur at an accelerated rate. If you notice that some part of your machinery structure is resonating, there are several steps you can take to correct this condition.
Be sure the mounting bolts and other anchoring hardware are properly torqued. Loose mountings often cause general looseness that mimics resonance.
- Vary the operating speed of the unit to change the frequencies of the exciting force(s).
- Add weight to the structure that is resonating. Changing the mass of the structure(s) also changes its natural frequencies.
- Brace the system where the movement is greatest. This not only changes the natural frequencies, but also increases the stiffness of the structures involved.
- Vibration in pipes may also be cured by adding bracing at the top of the separator, repiping the system in a different plane (increasing stiffness), or building a skid with concrete and grouting the anchor points.