Preventing Catastrophe

Almost 80 percent of all reported boiler accidents were attributed to two causes: low water cutoff, and operator error/poor maintenance. A low water cutoff condition occurs when the water level in the boiler steam drum drops below a pre-set safe level (as determined by the boiler manufacturer) and, in turn, shuts off the boiler. This condition, and the subsequent cause, should be investigated and corrected immediately. Failure of this safety control may result, at a minimum, in costly tube or vessel repairs, or, in the worst cases, catastrophic boiler and building damage and personnel injury or death.

Some common causes of low water conditions include:
•    Feedwater pump failure
•    Control valve failure
•    Loss of water to the deaerator or make-up water system
•    Drum level controller failure
•    Drum level controller inadvertently left in “manual” position
•    Loss of plant air pressure to the control valve actuator
•    Safety valve lifting
•    Wide variations or sudden changes in steam load

Avoiding the above conditions is critical to ensuring safe and reliable boiler operation. Maintenance, inspection, and operational logs are recommended and required by insurance companies.  These records not only help determine boiler performance trends, but also keep operators focused on the safe performance of the boiler and auxiliary plant equipment.  To this end, unnecessary boiler downtime (together with the loss of plant production) and lost time accidents are avoided.

The only way to avoid premature downtime and accidents (in the worst cases) is to make certain that operators and plant owners are committed to an on-going operational and preventative maintenance programs.  The Hartford Steam Boiler website www.hsb.com is a good resource.  Turning a blind eye to safe boiler operations puts operators, company employees, and equipment at unnecessary risk.

Edited from Nationwide Blog

Myths and Truths about Balancing

MYTH: “1x RPM is always caused by unbalance.”
TRUTH: Unbalance always causes vibration at 1x RPM.  However, 1 x RPM is not always caused by unbalance.  Many other problems can exhibit vibration at this frequency.  Examples are:  Misalignment, Bent shaft, bowed shaft, cracked shaft, eccentricity, open rotor bars in the motor, rubs, looseness, belt issues  and resonance. 

MYTH: “The run time and lifetime of the equipment can be extended by performing a balance job on the equipment.”
TRUTH: Problems such as misalignment, bad bearings, looseness, etc cannot be corrected by balancing the machine.  It is almost impossible to correctly balance a machine that has other defects affecting its performance.  Misalignment, bad bearings looseness etc should be corrected before attempting to balance equipment.

Pipe Strain is Soft Foot!

Soft foot means machine frame distortion. If you are missing shims under a foot and tighten the hold-down bolt until you have forced the foot down to the base, you will have distorted the machine frame. If you have severe pipe stress on a pump, and the anchor bolts are tight, chances are great you are also distorting the pump casing. Consider that if the pump’s anchor bolts were completely loosened or removed, the pump might be hanging in the air from the piping. So if you were now to tighten the anchor bolts, you would be forcing the pump down to the base and distorting it, just as happens when you are missing shims under a foot.

Shimming the feet will rarely solve the problem completely; rather, the correct solution is to eliminate the undesirable pipe stress. “Stress” is the force acting on something, while “strain” is the deflection or distortion resulting from the stress. A soft foot condition means you have machine frame strain, and pipe stress is just one of several examples of this. When the machine casing is distorted, the internal alignment between the bearings is changed and the shaft is deflected. This produces enormous stress on the bearings and increased vibration in your machines, resulting in premature wear and tear as well as loss of efficiency. Your seals and bearings will fail much faster. If a significant soft foot condition exists, a good alignment of the centerlines of the shaft rotation is almost pointless. The machines will still fail more quickly and lose efficiency. How do we diagnose and fix this?

The trick lies in knowing how to recognize that a pipe strain problem exists. The behavior of a machine with pipe strain differs significantly from one whose soft foot condition is caused by one of the more traditional shimming problems or unevenness of base or feet.

Any impact on the alignment of more than about 2 mils indicates a pipe strain problem that should be dealt with. Correcting pipe strain is a task for an experienced pipefitter who must see to it that connecting and torquing the piping should not move the machine from its rough aligned condition, nor distort its casing in any way. Proper pipe hanging techniques and a good knowledge of calculating and designing “Dutchman” spacers is essential.

Taken from Ludeca Blog

Understand shaft alignment fundamentals

Keeping your rotating shafts in alignment is a fundamental—and often overlooked—maintenance project. Alan Luedeking, the manager of technical support for Ludeca Inc., Doral, Fla., talked with Plant Engineering (PE) about some of the critical issues in shaft alignment, and how they affect safety, energy, and productivity.



PE: Maintenance personnel aren’t always looking for issues with shaft alignment. Are there some warning signs or performance cues that operators or maintenance staff should be on the lookout for that might indicate a problem?


Luedeking: Yes indeed. If you hear unusual noise or feel increased vibration, those are important warning signs that should not be ignored. All the senses should be involved. Smell your environment: chemical leaks, overheating grease, and unusual stains, all are diagnostic clues and warning signs. One should always be alert to everything in one's surroundings in the plant, for safety as much as for good maintenance. Most importantly, a proactive maintenance program should include condition monitoring-based strategies and solutions to prevent unnecessary repairs and unscheduled downtime in the first place.


PE: Let’s talk about the ROI of shaft alignment. When you’re out of alignment, what are the potential losses in terms of both productivity and uptime?


Luedeking: They are great. Besides the obvious risks of a broken coupling or shaft, misaligned shafts lead to increased radial load on the bearings with consequent great reduction in service life. Besides wear and tear, less obvious is that misalignment also results in increased power consumption, which several careful studies have shown to range as high as 10%, although more conservative estimates easily reach 4%. The efficiency of the machines is impacted, and product quality may be affected by the increased vibration resulting from misalignment.


PE: What’s the correlation between shaft alignment and energy efficiency? This would seem to be an overlooked area.


Luedeking: Funny you asked—I was just thinking about that in my previous answer. This aspect is often overlooked, and a savings of 4% on an energy bill of $50,000 a month easily justifies the best laser shaft alignment on the market in less than a year, without even considering the benefits of greater uptime, less repair expense, and time saved on the alignment itself.


PE: What are a few basics about shaft alignment that should be on the minds of operators and maintenance staff?


Luedeking: Safety and quality. Safety first, always. Make sure your machines are locked out and tagged out before you set up. When I say quality I refer to two things: the suitability of your laser alignment system for the task at hand, and the excellence of your alignment procedures. Are the machine bases properly designed, installed, and cared for? Always check for, analyze, and correct soft foot. Does your laser system help you do this?


Do you have jackscrews in place to move your machines? Do your millwrights have access to good-quality stainless steel shims? Are your machines under pipe strain? Can your laser system even measure that and help you find it? Are you considering thermal growth and dynamic load shifts in the positions of your machines? If not, you may be grossly misaligning your machines by aligning them to zero when cold.


Your laser system should let you input target specs, or thermal growth values at the feet, and calculate this growth from observed changes in temperature, or even measure and monitor this growth live as you run the machines! Only the best-quality laser system will let you do all of these things, thereby achieving better results and saving you time and money at every step.


Taken from Plant Engineering

Why air is compressed in Gas turbine engine ?

To understand why we compress air in a jet engine, one must first understand the goal. For an aviation gas turbine, the goal is thrust. The purpose of the turbine stage is to power the fan and the compressor. The thrust comes from moving air through the engine.
Now, we know why the fan is there. It's there to move the air. We get thrust from that. Now, the compressor's main purpose is to assist the turbine in energy extraction. By compressing the air, not only does it assist in combustion, it also helps in energy extraction by the turbine stage(s).
As for the temperatures, a turbojet will have significantly higher higher temperatures, as 100% of the flow goes through the turbine stage. As we mentioned, for a thrust-producing aviation engine, the goal of the turbine is to extract 'just' enough energy to power the fan/compressor stages. So, the flow still has a ton of energy, in the form of velocity and temperature. Exhaust temperatures can be well over 1000 degrees.
In a turbofan, as mentioned, some, if not almost all (in the case of the bigger engines) of the flow goes through the bypass. So, while the inner core flow will be hot, after the mixing region, it is relatively mild.

Proper Sizing and Installation Tips – Boiler Safety Valves

The safety valve is one of the most important safety mechanisms in a steam system. Not only are they required by code, but most importantly, safety valves provide a measure of safety for plant operators and for the equipment.
The American Society of Mechanical Engineers (ASME) governs the code that establishes the requirements for safety valves, therefore it key that all plant personnel are familiar with current codes that apply to their system.
The following sizing guidelines and installation tips listed in Process Heating Magazine and thought the information would be useful to pass on. We hope that this enhances your knowledge and understanding of safety valves.

Sizing Guidelines
The two major considerations for safety valves are proper sizing and correct installation. The following tips address safety valve sizing.

• It is suggested that the setpoint selected for the safety valve provide a differential of at least 20 percent between operating and set steam pressures.
• When considering installation of a safety valve downstream of a steam pressure control valve, the total capacity of the safety valve at the setpoint must exceed the steam control valve’s maximum flow capacity (the largest orifice available from that manufacturer) if the steam valve were to fail to open. The inlet steam pressure to the valve must be calculated at the maximum safety valve setting of the steam supply source, not the nominal operating pressure.
• It is important not to oversize a safety valve. Bigger is not better in this case because a larger-than-required valve could cause chatter, leakage and premature failure.
• Many times, a single safety valve is not possible due to high capacity, physical limitations or economic considerations. An acceptable alternative is to employ multiple safety valves on the same system. The valves should be of the same setpoint and the capacities must be equal to or greater than the rating of the equipment. Additionally, the vent pipe must be sized to account for the venting capacity of all of the safety valves fully opening at the same time.
• The set pressure of the safety valve should be set at or below the maximum allowable working pressure (MAWP) of the component with the lowest setpoint in the system. This includes steam boilers, pressure vessels and equipment, and piping systems. In other words, if two components on the same system are rated at different pressures, the safety device protecting both of these devices must be set at the lower of the two ratings.

Installation
Once sizing has been properly determined, proper installation is the next crucial step to ensure safety. There are several points to consider when installing the safety valve.

• The steam system must be clean and free of any dirt or sediment before commissioning the steam system with a safety valve.
• The safety valve must be mounted vertically with the valve’s spindle in the vertical position.
• The inlet steam piping to the safety valve must be equal to or larger than the safety valve inlet connection.
• There should be no intervening shutoff valves located between the safety valve inlet and the steam component that could permit the safety valve to be isolated from the system.
• Drains or vent openings on the safety valve should not be plugged or capped. They are on the safety valve for a reason.
• Safety valves are set, sealed and certified to prevent tampering. If the wire seal is broken, the valve is unsafe and should not be used. Contact the supplier immediately.
• For multiple safety valve installations using a single connection, the internal cross-sectional area of the inlet shall be equal to the combined inlet areas of all the safety valves.
• All safety valves should use a drip pan elbow on the outlet. The drip pan elbow changes the outlet of the safety device from horizontal to vertical. Installation of the drip pan elbow has its own guidelines, which should be researched and addressed to meet the needs of each application.
• Never attach the vent discharge piping directly to the safety valve. This would place undue stress and weight on the valve body. Also, the safety valve vent pipe may not touch the drip pan elbow.
• The drains on the drip pan elbows are to direct condensed vapor and rain safely away to the drain. Do not plug these openings.
• Steam will not escape from the drip pan elbow if the vent line is sized correctly.

Vent Piping
There also are some important considerations when it comes to the vent piping of the safety valve and the steam system.

• The diameter of the vent pipe must be equal to or greater than the safety valve outlet.
• The vent line should be sized so that back pressure is not placed on the drip pan elbow.
• The length of the vent pipe should be minimized where possible.
• The discharge outlet of the vent pipe should be piped to the closest location where free discharge of the safety device will not pose a safety hazard to personnel. For a roof-line termination, the vent should be no less than 7′ above roof line. The top of the vent line should be cut at a 45° angle to dissipate the discharge thrust of the steam, prevent capping of the pipe and to visually signify that it is a safety valve vent line.

Fig. 1: Typical safety valve designs

Taken from Nationwide Boiler Blog



The Function of the Volute

Misunderstood pump element serves to minimize mixing loss

It is a common misconception in the U.S. pump industry that the function of the volute is that of a diffuser: to convert velocity into pressure. The McGraw-Hill scientific dictionary states that a volute is “a spiral casing for a centrifugal pump…designed so that speed will be converted to pressure.”


Function at Best Efficiency Point (BEP)

Figure 1. A single-volute casing maintains a constant velocity and uniform pressure around the impeller only at BEP.

Figure 2. Hydraulic radial thrust for volute casings.

It is understandable that such a concept has been adopted because the volute has an increasing flow area as it wraps around the impeller, similar to a diffuser, but it is not the purpose of the volute to be a diffuser. Its function—when the pump is operating at the best efficiency point (BEP)—is to keep the velocity constant around the impeller so that mixing losses are minimized. To achieve that function, the area increases so as to accept the additional flow exiting the impeller, which exits the impeller all around the outside diameter (OD)—360 degrees. The pressure surrounding the impeller is uniform, resulting in zero hydraulic radial thrust on the impeller.
 
Performance with Restricted Flow
When the flow from the pump is restricted, forcing the pump to operate at a reduced capacity, the flow from the impeller is reduced, and the volute does act as a diffuser, creating an increasing pressure from the cutwater all the way around to the casing throat. The maximum pressure rise occurs at shut-off (zero flow). As shown in Figure 1, this rise in pressure around the impeller creates a radial thrust on the impeller that pushes the impeller in a direction approximately 90 degrees downstream from the cutwater. As shown in Figure 2, the maximum thrust occurs at shut-off.

Performance with Excess Capacity
When the pump is allowed to operate at a capacity that exceeds the BEP, the result is just the opposite. The velocity around the impeller increases, from the cutwater to the throat, causing a drop in pressure. This results in a radial thrust that pushes the impeller in the opposite direction, approximately 270 degrees downstream from the cutwater, as shown in Figures 1 and 2.
 
Taken From Pump-Zone