페이지 정보작성자 최고관리자 댓글 0건 조회 109회 작성일 21-02-15 16:47
DESMI’s Sales and Application Manager in China, John Nielsen has a message for ship owners all over the world: “Be aware of how your pump systems are installed. Ensure good workmanship!”, he says. “That’s the bottom line. There are too many outfitting crews lacking practical experience.”
“Ship owners must thus make sure their site teams follow key guidelines for proper pump installation. Because most of the time, a pump breakdown is not due to a faulty pump”, John says.
“Ninety-nine percent of the time, a breakdown is linked to a lack of installation knowledge during installation. Or missing parts. Or incorrect operation–like starting the pump without liquid inside–that’s a killer. It is linked to a lot of things”, he says.
With several years at DESMI and more than 20 years as a shipbuilder, John Nielsen knows what he is talking about. He offers a list of “do’s and don’ts” in pump installation for ship owners, superintendents, site teams and shipyards. “You need to be aware of these things”, he says. “A pump is just a component that will do exactly what it is designed to do. But if it’s not installed properly, then you can destroy it. It’s a bit like Murphy’s Law: If something can go wrong, it will. And in the end, the ship owners will have to bear the costs”
A solid foundation
Solid pump foundations are at the top of John’s list. “It is extremely important that these are done correctly. The foundation has to be supported so there’s no chance of wobbling.” In other words, do not install the pump on a soft plate–a steel plate with no or missing support below. “There’s an old shipyard saying: You need the same amount of steel below the plate as you have on top of it.”
If the outfitting team wants to use weighted or spring-loaded supports, they must first check the design with DESMI or the steel engineer. “It is not up to the individual outfitting team to decide”, John says. “Most pumps today are vertical. And two-thirds of a pump’s weight is actually the electrical motor. Some pumps are four and a half tons meaning the electrical motor is three tons. This all calls for very good foundations to avoid vibration, etc.”
Pipes and valves
There are standard ISO rules for pipe routing, but these can be tricky to follow onboard a ship due to limited space. “So that means you have to think a bit alternatively,” John says. On a ship, make sure that the pipe design allows the liquid flowing to the pump on the suction side at a maximum velocity of 1 meter per second(m/s). This is a rule of thumb so that you avoid creating water vortex or cavitation that will eventually destroy the pump.
And then there are valve choices. On ships, a big number of valves are butterfly valves, mainly because this is an inexpensive valve type. But a butterfly valve only works in the pumping direction. This is why there are always a non-return valve in series with the butterfly on pumps pressure side. Otherwise, if pumps are working in parallel and one pump is dormant without installed non-return valve, the active pump will pump liquid to the other pump instead–no matter butterfly valve open or closed.
Furthermore, it is important that the valves are full bore(full port). This means that when the valve is open, it has the same diameter as the pipe. Don’t use valves that are slightly smaller in diameter than the pipe. This will create small vortexes inside the pipe and will eventually destroy the pipe and the valve.
The sea chest line
Make sure the crossover line supplying water from the ship’s sea chest is sized properly. Sometimes, John Nielsen says, it is designed too close to the limit. This affects the flow rate and can cause pumps to lose suction. If this happens to the vessel’s main sea water cooling pump, this could lead to shutdown of the main engine and loss of propulsion power putting the vessel and crew at risk.
At full load on the crossover, the liquid velocity should as a rule of thumb be maximum 0,7m/s. This to ensure a steady flow to pumps suction lines.
Materials and corrosion
It is also important to plan carefully for the pump material used, depending on the liquid it is pumping and conditions around it. Material choices for pumps include bronze, nickel aluminum bronze(NiAIBz), gun metal, cast iron, cast steel, stainless steel 304, stainless steel 316, and duplex. “All of these materials have each their own advantages and disadvantages. It’s important to consider these”, John says.
DESMI typically uses stainless steel 316 or duplex for alkaline or acidic liquids, for example. “But you have to handle this material very carefully. You need to know what you’re doing. Otherwise they can corrode.”
DESMI designs its seawater pumps typically of NiALBz for a designed sea water temperature of 32°C. Sea Water temperature above that becomes much more corrosive. Likewise, cast iron is not a good choice for seawater pumps in warm areas. “If you’re operating in Greenland, then a cast iron pump can last a long time, but not if you’re working in the tropics.”
For low temperature freshwater pumps, the DESMI design is using cast iron housing and NiAlBz impellers, designed for a water temperature of 36°C. “The problem with fresh water on a ship is that you need additives to remove oxygen as to avoid corrosion/rust. But there are many different kinds of additives and some of these make the water very viscous, and as result water can be leaking everywhere. Another problem with additives is that these chemicals change waters pH values. This is also something that needs to be considered when selecting materials.”
Be aware of the potential for galvanic corrosion(also called bimetallic corrosion) when considering materials. This is a phenomenon that occurs when two different materials are put together–like copper and iron in salt water. Salt water will act as an electrolyte. “It’s a slow killer”, John says. “It takes a long time before you find out it is a problem.”
”It is the same principle as used for protection steel with zinc anodes.”
In the case with copper and iron–the iron will slowly move to the copper.
“I think many people have experienced that calcium is building up inside pipes, etc.–this is also due to galvanic element effect.”
John’s main advice about material choice is to look carefully at what is supposed to be used in different conditions and listen to the experts.
“We see many cases where customers specify bronze. And yes, we can use bronze, but we prefer to use nickel aluminum bronze, because it is more rigid. And it’s also better for avoiding galvanic element corrosion. So, this is the DESMI choice.”
“One last word in this materials section, and that is related to how to increase the pump efficiency. Most pumps have an efficiency of 75-80%”, John says. “Some ship owners require 80-83%. And this demands more than just an update of the mechanics. While there are many ways to do this, coating a pump internally with a glass-like coating is the most common method. It is also possible to use an electrical control system to increase the efficiency. More and more ship owners are asking for the higher efficiency.”
John has also made a short list of common mistakes–practical issues that must be considered. “These are small issues around the pump. Although small-they still need to be considered:
•Make sure pipes are not under tension when you connect these to the pump.
•Make sure pipes are compensated for expansion and contraction due to different liquid temperatures.
•Make sure you have the means to lift the pump safely after it is installed in the ship. Otherwise maintenance can be difficult–some pumps are very heavy!
•And make sure you have the required tools and parts handy for pump and system maintenance onboard.
It is super important that traditional electrical installations follow the relevant classification society rules and standards from the International Electrotechnical Commission(IEC). This includes factors like correct installation, correct cable dimensions and correct cable temperature classes. “This all has to be checked”, John Nielsen says. “And you must have self-extinguishing cables in case of fire. Cables must also be halogen-free. Big cables or small wires–they all have to comply with the same rules and regulations.”
Ground connections must be done carefully, with the correct cable dimensions. “No cheating”, John says. “That means, bottom line, the size of the ground connection wire must almost be the same as the main supply wire. So, if you’re using a 3x50 mm2 power cable, then you must as a minimum use a 1x35mm2 cable for ground connection.”
A rule of thumb is to use the same wire cross section in grounding wires as used in the power supply cable to the individual components.
John would also like to urge consideration of where the potential ground connection is made. “There must be no potential electrical differences between the grounding point for the same equipment.
That means that if you have a motor starter, then the ground connection for that motor starter driving that pump is the same spot as where you put the ground connection for this electrical motor. This is also known as equipotential grounding. Because if you put them 10 meters apart, you can be very sure that there’s a difference in the electrical potential”.
He says, “there’s a common belief that just because you weld something to a steel plate, there are no differences in the electrical potential. But they are huge. Just the welding itself makes a difference.”
For electrical protection of pump and motor, there are two settings that are critical:
The molded case circuit breakers(MCCBs) must be correctly adjusted and correctly installed. Settings on the MCCB is the maximum current stamped on the electrical motor.
Setting on the thermal relay is according to the pump’s non overload power, which is given from the pump’s test sheet.
So, the MCCB protects the installation and the thermal relay protects the pump.
The advice, and rules and regulations about using a frequency converter follows the above recommendations for traditional electrical installations. But you must take extra considerations, since the frequency creates electrical noise that must be managed. “Make sure you at least have a common mode filter on a frequency converter when you do an installation. Larger installations may require du/dt filter.”
You must also make the correct adjustment of maximum current for a frequency converter–the same as you would do for the thermal relay in a traditional starter.
And then, when you want to use a frequency converter to operate your electrical motors, then you should seriously consider to increase the temperature class of the electrical motors to be one temperature class higher. “Normally, the choice is Class F, which is 135°C.
But when operating with frequency converters this will make your motors warmer due to the switching frequency on the motor supply line. So, we recommend: go up to Class H, which is 150°C. That will give you a bit more room to maneuver. And pricewise, it’s small money.”
The cooling fan and IE2
Furthermore, make it standard that you want to measure the winding temperature inside the motor. That means that your motors should come with PT100 sensors embedded. Then we can measure it directly. And also, as this motor is often running at a very different RPM, then make sure you have an electrically driven cooling fan on top, instead of a standard mechanical one. Because when you run the motor at a low RPM, you don’t have the same cooling capacity as you do when it goes at normal speed. But if you put in an electrical fan, you will have the same amount of cooling, no matter the speed of the load.
John Nielsen says, “when using a frequency converter, consider seriously using normal rated IE motors–or standard efficiency motors. The reason for this is that you don’t gain anything by increasing the IE class on the electric motor, if it’s connected to the frequency converter. Because the summarized efficiency of the grid will stop at the frequency converter, meaning the grid’s efficiency on the supply line is determined by the frequency converter. And the price difference between IE2 and IE3 or IE4 is significant.”
More monitoring advice
In addition to monitoring electrical motors’ winding temperature, John says, “it is worth monitoring bearing temperatures – especially on important equipment. Furthermore, consider monitoring vibration levels – both at high and low frequencies.”
“It’s not unusual that we see a vibration level of 10 millimeter per second (mm/s). And the limitation for the pump–if we are really stretching it–is 7 mm/s. In case of an excessive vibration level, you might have to stop the pump to avoid damage to the pumps or pump parts. Remember, two-thirds of the pump’s total weight is still the motor–sitting on top of the pump casing, bearings, etc. So, if you have a three-and-a-half-ton pump motor literally jumping 10 mm/s, this is just like a huge hammer, knocking down on some small bearings somewhere. So–it’s just a matter of time before this pump stops by itself.”
There is also leakage monitoring. For big pumps that have grease-lubricated bearings, if the shaft seal starts leaking, very often it will leak straight down into the bearing. And grease and water are a bad combination. So, in order to avoid this, consider installing leakage detection sensors.
■ Contact: www.desmi.com