In this description, we give a clear explanation of the advantage of the OH5 inline pump design over the OH2 centreline mounted design.
Especially in the oil and gas industry, centreline mounted OH2 pumps or the inline OH5 pumps are widely used. They are the workhorses of the industry and you see them everywhere. These two pump types are both built at Rodelta, and fully comply with all the requirements of API610 11th edition. (no exceptions) But why do some customers (Shell) choose an OH5 over an OH2?
To give you a good insight, we have compiled a list in which the advantages of OH5 pumps over OH2 pumps are highlighted. At the end of the summary, we also highlight some of the restrictions on the OH5 design that require a different choice. Each situation is illustrated with a visualization to give you a clear picture.
Required floor space
Advantage OH5:
The OH5 (ETL) has a very small footprint and is therefore popular in places where space is at a premium. Especially on vessels and/or off shore platforms or in places where accessibility is limited.
Seal accessibility
Advantage OH5:
The OH5 pump offers more freedom with regard to connections to the seal. With the OH2 version, the baseplate always provides limited accessibility and makes connecting the seal pipes more difficult. The connection of the bearing housing to the pump also limits the possibilities.
Standardised pump press and suction connections.
Advantage OH5:
The pump housing of the OH5, with its discharge and suction flange dimensions, complies with British Standard 4082. (BS4082) The footplate of the OH5 complies with the Shell DEP dimensions for in-line pumps.
For the OH2 centreline mounted pumps there is no standard for the discharge and suction dimensions. This means that: When the pumps are replaced by another brand, the pipework and the foundation will probably not fit and will have to be modified.
The BS4082 for OH5 pumps even covers the U-turn pumps with its dimensions. Rodelta also supply these solutions.
Installation template option
Advantage OH5:
A major advantage of the ETL OH5 pump type is the possibility that the pump installation work can be started at an early stage. By pouring of the sole plate and the welding of the pipework at the location before the actual pump is on-site. The overall lead time can be significantly reduced. This also creates the possibility of absorbing project overruns, as activities can take place in parallel rather than one after the other. (More info: here) This is possible because the foundation only supports the pump and has no alignment function for the motor.
This is in contrast to an OH2 pump. There the foundation frame is an essential part for the alignment. Upfront on site pouring of the frame creates an uncontrollable alignment of motor and pump. Normally this activity take place by the manufacturer in the factory. The frame dimetions are also dependent on the dimensions of the pump and motor, which is not the case with the OH5. Production delivery time of the baseplate will therefore be longer which eliminates the benefit. In addition, one would even have to produce two frames if they also had to perform one site test.
Alignment
Advantage OH5:
The number one cause of seal problems is caused by misalignment of the pump and motor. An OH5 is designed with a fitting edge construction that ensures that the parts are always re-centred. (This is realised and guaranteed in the factory).
An OH2 also has a fitting edge construction for the pump but not for the motor. The motor must be aligned on-site after the pump has been installed. The responsibility is for the technician on site. Also after maintenance work, it is always necessary to check the alignment. In the case of a Rodelta OH5 pump, this is guaranteed in the design.
NPSH available
Advantage OH5:
OH5 pumps have a more favourable NPSH available due to their design. This is certainly 30% or more compared to the same pump in an OH2 version.
Efficiency and discharge and suction dimensions
Advantage OH5:
For OH5 pump, the suction and discharge diameters are the same. These are different for OH2 pumps. In an OH2 pump, the suction diameter is larger than the discharge. Which is remarkable. This means that the same amount of liquid that enters the pump through the suction must leave through a smaller diameter. This is only possible if the liquid speed increases. But if the speed of the liquid at the inlet is already the miximum speed for the system piping (otherwise pipe cavitation will occur), than the speed at the outlet must be reduced after leaving the pump. This is mostly done by placing a reducer. However, this is destroying energy. Which, if you want to compare apples with apples, must be subtracted from the OH2 pump efficiency.
An OH5 is so designed that the hydraulics only increase the pressure and not the speed of the liquid.
Weight of pump system
Advantage OH5:
An OH5 pump is on average 25% lighter than an OH2 pump. This in combination with its small footprint makes it extremely suitable for use on offshore platforms and on LNG vessels. However, it must be said that an OH5 pump casing is heavier due to the suction box construction than an OH2 pump casing, which is disadvantageous with regard to the cost price of ecotic materials. But this can be compensated by the advantage weight. (However, this must be reviewed per circumstance).
Thermic influences
Advantage OH5:
Large temperature differences in the pumped medium cause expansion forces on the pipework. These forces in an OH2 pump are at right angles to each other, resulting in a resultant force that may cause displacement or deformation. As the motor in an OH2 pump is fixed on the baseplate, separate from the pump housing, the alignment can be affected. In the case of an OH5, the pipe connections are aligned with each other. Any forces arising from pipe temperature differences have no possibility of affecting the alignment of the motor to the pump.
Expansion of the pump shaft due to heat also affects the coupling with an OH2 pump because the motor is fixed on the other side on the baseplate. With an OH5, the expansion grows along with the pump shaft as the motor stool is also heated.
Drain seal leakage
Advantage OH5:
Any leakage from the seal in the Rodelta OH5 is collected in the seal chamber. This seal chamber has a drain outlet to the outside on the pump cover. A pipe can be attached so that the liquid can be discharged in a controlled way.
On the OH2 pump the leakage from the seal is collected on the base plate after which it is drained to the drain pipe of the base plate. This solution is less controlled and the risk of contamination of the environment is therefore larger.
Replacement of pull-out unit
For safety reasons, in practice it is often the case that in both pump situations the motors are disconnected electrically before starting the job.
To replace an OH5 pullout unit, 16 bolts are removed. The pullout unit can then be lifted. Lifting is easy because the unit is perfectly balanced by the existing lifting lugs on the motor. Any residual liquid left behind after the pump has been drained will fall back into the pump casing when it is lifted out.
To change the pull-out unit on an OH2 pump, the coupling protection cap must be removed. Then the spacer between the coupling halves can be disassembled. The lifting straps can be inserted and the 16 bolts on the pump cover can be removed. It is important to ensure that the pull-out unit is lifted horizontally and is in balance. If it is not balanced, the pullout unit may collapse and causing damage to the seal surface and fitting edges. The small amount of liquid which is often still present around the seal and impeller falls onto the baseplate. Take care that this does not cause any damage by slipping or environmental hazards.
Looking at the two operations, we can say that the exchange of the pullout unit in an OH5 is faster, more controllable and cleaner than in an OH2.
Advantage OH5:
In the case of an OH5 pump, gravity is in the longitudinal direction of the shaft. In the case of an OH2, gravity is at right angles to the axis. Especially when the pump is running out of speed and the Lomakin effect disappears, all tolerances in the bearings and the bending of the shaft are in one direction with an OH2 pump. Therefore, this design takes more account of the clearance at the impeller wearing.
This can be more accurate in an OH5. Because in this situation all forces and clearance are in the longitudinal direction of the shaft, and thus do not affect the seal ring clearance. And can therefore be smaller. And as you know, a smaller clearance means less fluid loss over the seal, so better efficiency.
Same function less parts
Advantage OH5:
For the same function, less parts are used in an OH5. The baseplate, for example, is not required any more (1). The coupling with spacer and keyway (2) are not required any more. The coupling guard with attachment, are not required any more (3). The bearing housing with the seals, cooling vane, shaft nuts and all provisions as there is the sight glass, the oil filler, the vibration sensors, ball bearings and drainage plug are not needed (4). But with this also all the adjusting, broken or skewed oil filler problems are over. Also the not on time refill of oil, dry running of the bearings, and the contamination of the lubricating oil belong to the past.
Besides these mechanical aspects, it also contributes to the United Nations Sustainable Development Goals (SDG), which companies are required to report on in their Corporate Sustainability Reporting Directive (CSRD). This is a mandatory report that companies will have to present annually starting from 2024, regarding their sustainability policy. This is because all components that are not used in an OH5 pump compared to an OH2 pump do not need to be extracted from earth materials. The production of these materials and casting them is also not necessary. Mechanical processing is no longer required, and transportation, etc., is also no longer necessary. All in all, a beautiful saving that will be reflected in your SDG (Sustainable Development Goals).
limitations
However, the OH5 also has some limitations. One of them is the maximum liquid temperature. This may not exceed 250 degrees Celsius. If you have an application that needs to run at higher temperatures you can select the OH3 (ETLS) from Rodelta. This is a vertical oil lubricated diffuser pump with the same standardized pump housing dimensions.
Another limitation is that the seal system must be separate from the pump. Rodelta can supply the pump also on an enlarged floor plate so that this is still possible.
We sometimes hear that it is more difficult to replace the seal in an OH5 pump. We strongly oppose this. Exchanging the seal is the same or maybe even easier than with an OH2. We always offer the possibility that one of your mechanics gets an internal training in how to do this the easiest way.
Finally, we would like to say that the points mentioned above ensure that this pump has a very long main time between failures (MTBF). And help you to carry out maintenance in a controllable way.
