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ОглавлениеTo begin a comprehensive machinery assessment, you need to know everything you can about the machine in question: details of its construction, its history, how it’s being operated, etc. Then, you need to collect some field data and analyze it. Finally, you have to make a decision considering all the field analysis results and pertinent factors. This sounds easy, but in practice machinery assessments are challenging and sometimes frustrating. To simplify the process, we will break the steps down in to manageable steps.
In a nutshell, here are the basic recommended steps that will help you in your quest:
Step #1. Ask a lot of questions.
Step #2. Collect equipment information.
Step #3. Collect machine field data.
Step #4. Make a decision.
Remember to ask questions if you have any doubts. Operators, co-workers, and supervisors may have some additional knowledge or insights into the problem at hand. The help of the original equipment manufacturers (OEMs), co-workers, consultants, etc. should all be enlisted if you are unsure of how to proceed or if you need additional machine information.
Remember:
The information in this reference guide should be considered as only one facet of a comprehensive assessment methodology—not the final word. The guidelines contained in the following pages must be used judiciously, ensuring that all the collected data is accurate and trustworthy and that ample machinery information is at hand.
Here’s a basic outline to help you navigate through the machinery assessment maze:
What questions should I ask operating personnel?
1.Which machine element is acting up?
a.Driver
b.Driven
c.Piping/attachments
d.Auxiliary equipment
2.What’s the problem?
a.Vibration
b.Temperature
c.Erratic flows or pressures
d.Pulsations
e.Loss of flow or pressure
f.Frequent machine trips
3.Did the problem develop suddenly or gradually?
4.Is the problem intermittent?
a.Does the problem come and go with flow changes?
b.Does the problem come and go with pressure changes?
c.Does the problem come and go with suction level changes?
d.Does the problem come and go with speed changes?
e.Does the problem come and go with fluid property changes?
5.What is the machine’s status?
a.New installation
b.Recent modifications
c.Recent repair
d.Recent preventative maintenance activity
e.Has it been running OK for over 90 days, but has recently started acting up
f.Has it been recently idled
g.Are you operating at off-design conditions?
i.High or low flows
ii.High or low pressures
iii.High or low speeds
6.What is the machine’s history?
a.What’s the historical mean time between repairs (MTBR)?
b.What are the common failure modes?
c.Are there any known design problems?
d.Are there any known operational issues?
What machine information do I need before getting started?
1.Machine spec-sheets
2.Machine drawings
3.Machine trend data
4.Machine test stand data, if available
5.Machine’s maintenance history
What analysis equipment do I need?
1.Vibration sensors and analyzers
2.Temperature gun
3.Pressure gauges
4.Dynamic pressure transducer and analyzers
5.Tachometer or strobe light
6.Strain gauges for piping studies
7.Sample bottles for oil analyses
8.Camera if permitted
Making a decision:
Once you have collected some field data, you will need to make a decision. Your choices are:
•Do nothing.
•Monitor: Continue monitoring as scheduled, or increase monitoring intervals and/or points.
•Test and assess: Vary operating conditions and observe how equipment responds.
•Switch to spare, if available.
•Plan a shutdown for balancing, alignment, oil change out, piping modification, etc.
•Plan a shutdown for repair (e.g., full machine overhaul if needed).
•Shut down immediately for balancing, alignment, oil change out, piping modification, etc.
•Shut down immediately for repair.
The Potential to Failure Time Interval
For most classes of rotating equipment, there is usually a time interval between the time a failure can first be detected until the time a catastrophic failure occurs. This time interval was given the name the Potential to Failure, or P-F, interval by J. Moubray. Potential to Failure (P-F) curves graphically display the failure time cycle and measurement techniques that can be used to detect machinery failures prior to realization of a catastrophic failure.
Proactive strategies, such as vibration analysis, lubrication analyses, and IR imaging all focus on keeping machines on the far left side of the P-F curve, or early in the failure cycle (Figure 4.1). The times in this chart are for illustration only and should not be considered valid for all cases. The ability-to-detect-early failure is a hallmark of top performing maintenance programs. Failure to maintain vigorous predictive maintenance programs will doom maintenance organizations to operate reactively instead of proactively.
It is important to understand your machine’s P-F curve when making decisions on how to proceed. For example, if you are early in the P-F interval (vibration), then there is probably ample time to perform further tests and/or properly plan a repair. However, if the machine is generating audible noise, there is probably significantly less time, e.g., a week or two, to react before failure. However, if the machine’s is hot to the touch in the vicinity of the bearing, or bearings, or there is visible smoke, the game is over—there is nothing left to do but prepare for an emergency shutdown. It may be useful to generate a P-F curve similar to the one shown in Figure 4.1, as a visual aid to management, so that they can better understand where you think you are in the failure cycle and how much time there is left until failure occurs.
Figure 4.1 P–F Interval Curve
Takeaway: When making a decision, understand where you are in the P-F interval
Before making an informed decision, you need to have answers to the following questions:
a.Do you have enough machine information to effectively analyze it? If not, continue your research.
b.Based on this guide or the equipment manufacturer’s guidelines, are any of the measured machine variables at dangerous or destructive levels? If the answer is yes, your choices are to either shut down, switch to the spare, or attempt to change operating conditions to see if things improve.
c.Is there sufficient field data to make an informed decision? If not, collect more data.
d.Is this machine spared? If it is spared and conditions warrant it, switch to the spare.
e.Is this machine critical enough to warrant additional, more sophisticated tests? This is an economic decision.
f.Is there time to run additional tests? As long as levels are at manageable levels and the economics warrant it, you can continue testing. A good analyst knows when it is no longer economical to continue testing.
g.Can the machine be shut down temporarily for balancing, alignment, oil change out, piping modifications, etc.? This may be your only option for unspared machines. Note: Make sure any maintenance-related outage is well planned before shutting down.
h.Are there sufficient analysis points, such as pressure taps, oil sample points, etc., to get a clear picture of what is happening? If not, the machine may have to be shut down to add these points at some convenient time in the future.
Now that you have all your machine information and machine data, and have answered the questions above, sit down with the machine owners/operators and develop a plan of action that everyone agrees with. Be sure your decision team includes representatives from operations, maintenance, reliability, and management.
