Bed-Plates for Rotary machines testing rigs

[jashprecision]

It is found that in majority of the cases Machines Testing technicians are seldom part of design or selection process for the testing rig as a whole or test rig elements, though he may be using the system. He is a person who is responsible for testing of rotary machines and many a times he encounters testing problems and yet he is to arrive at conclusive results. As much as feasible, it is important that he is part of design / selection of the testing rig system as whole. If not, at least he must be aware of the functions of each testing rig element to correctly analyze the problems he may encounter during testing.

In the context of testing rig design for rotary machines the system would necessarily include;
(a)A drive machine
(b)A driven machine
(c)A coupling device
(d)A bedplate or base plate or base frame, and
(e)A concrete mass foundation and isolation thereof

For designing of test rig and foundation thereof machine data / details for all the five above are necessary. The complete knowledge of excitation forces, associated frequencies and load transfer mechanism from the machine to the foundation is a must for correct evaluation of dynamic response. A close interaction between the bedplate suppliers, foundation designers and rotary machine supplier as well as appreciation of each other’s limitations, therefore, is essential.
To understand the testing rig system as whole it is desirable to note and understand the following.
1.Though the supplier for all the three machines may be a single agency, invariably manufacturer would be different for each machine.
2.Each machine according to its footprint has its own base frame and bolting arrangement with the foundation.
3.Each machine is balanced independently as a separate unit.
4.Each machine rotor will have its own critical speed.
5.When these machines are coupled together and supplied or installed as a set, the data for individual machine may not fully hold good for the coupled machines. It may need appropriate correction / modification. At times, these machines may be mounted on a common base frame instead of their individual base frame and that will reflect as a change in the mass and Center of Gravity location.
6.The equipment or system drawings and data sheets supplied by the machine manufacturer do provide a host of information about the machine and out of these, only the information required for the bed-plate and foundation design needs to be selected. In certain cases, some data may have to be processed for design purposes. Thus for a properly selected bedplate, designed foundation, careful determination of those design machine parameters that influence the response of test rig system as whole becomes essential.
The available machine data, therefore, needs to be suitably converted and translated in to design machine parameters for use in selection of bedplate and machine foundation design.
It is general practice in the industry to pay more attention only to those machines that do not perform well. More often than not, for every malfunction one keeps on trying modifications in the rotary machine like better balancing, replacing bearings etc. till satisfactory results are achieved. It is worth noting that when malfunction occurs the cause may not be the machine alone but it could be the base plate or foundation too. In certain cases the desired results could be achieved by correcting the source, which may be other than the rotary machine.

contd..

[jashprecision]

Mechanical components of the rotary machines testing rigs are principally (a) Cast Iron Bed-Plate or Base Plate or Test Bench (b) Vibration Isolation or Dampening and (c) Concrete mass foundation underneath Bed-Plate or Base Plate.

(A) CAST IRON BEDPLATE OR BASE PLATE:

Cast Iron Bed-Plate or Base Plate is vital element in testing rigs for rotary machines. Hence, it is fundamental to select the right kind of Bed-Plate that is the foundation for quality of rotary machines. Gray cast iron is most preferred material of construction for Bed-Plate or Base Plates since they have excellent vibration dampening characteristics over other metals. Cast Iron Bed-Plate are designed such that their natural frequencies are very low.

Cast Iron Bed-Plate or base plates are available in different sizes, load carrying capacities and configurations. They can also be tailor manufactured to meet specific requirements. They can also be assembled together to create large surface areas as per the requirement. Such assembled Bed-Plates are leveled, aligned and interconnected with each other to obtain flat and plane surface area.

The basic features on Bed-Plate or Base Plate are selected based on their application as part of a testing rig. Following technical details are essentially to be provided by the User / Buyer that helps manufacturer / supplier to suggest the appropriate Bed-Plate

(1)Rotary machines are to be tested in horizontal position or in vertical position or both. E.g. foot mounted and / or flange mounted electric motors
(2)Power rating of testing machines. E.g. H.P. / kW / Torque etc.
(3)Normal operating speed range
(4)Critical speed range
(5)Maximum permissible vibration amplitudes on machines under tests
(6)Maximum anticipated total loads, both static and dynamic
(7)Testing equipment dimensions
(8)Testing equipment mounting arrangement
(9)Center of Gravity locations in the testing equipment in X, Y and Z directions
(10)Equipment foundation edge distance requirements
(11)General arrangement drawing showing plans and elevations for testing equipment
(12)Testing equipment foundation outline drawing, plans and elevations

Bed-Plates or base plates can be of two distinct features. They may be either anchored on to concrete mass foundation or they may be permanently grouted on to the concrete mass foundation or stable, non-flexural rigid concrete shop-floors having requisite load carrying capacity.

We recommend anchored type Bed-Plates since they can be removed for shifting to different locations when required or when re-leveling and re-conditioning is necessitated during their use over a period of time. Here we may say that Bed-Plate anchoring and leveling elements are important elements of any Bed-Plate or base plate for testing rigs. Permanently grouted Bed-Plate or base plates can not be removed from their foundation nor is re-leveling nor reconditioning possible. Permanently grouted Bed-Plate / Base Plates or test benches are economical then anchored type Bed-Plates or base plates.

(B) VIBRATION ISOLATION & DAMPENING:

For electrical machines having rigid rotors IS0–1940/1 recommends balance quality grade (vibration amplitudes) represented as “Gr” (e.g. G0.4, G1, G2.5, G6.3, G16, G40 etc.) where the letter “G” is used as notation for Grade and “r” is the number (in mm/sec) that represents product of eccentricity in mm and rotation speed “ω” in rad/sec.

The compatible vibration isolation elements could be interfaced between Bed-Plate and concrete foundation and / or between concrete foundation and soil around concrete foundation. The vibration isolation can be of two types viz; (a) passive vibration isolation or (b) active vibration isolation.

Continued ...

[jashprecision]

Vibration isolation is required to reduce the transmissibility of the exciting forces generated from the rotary machines to the foundation and vice-versa. In broader sense vibration isolation is done for the following:

(1)Control of transmission of dynamic forces from the testing rotary machine to the foundation and thereby to the adjoining structures, plants and machinery.
(2)Isolation of equipment from the vibration effects of the adjoining structures, plants and equipment.
(3)Isolation from external forces like earthquake shocks, blasts etc.

(C) CEMENT CONCRETE MASS FOUNDATION:

Another important element is design of cement concrete mass foundation for testing rig especially when vibration levels of running rotary machines at the time of shop testing are to correlate with that of installed rotary machines at customers’ plants. Any discrepancy observed in these values of vibration levels guide us to identify the problems and take corrective measures.

As a rule of thumb the mass of well compacted and well cured cement concrete block (density ≈ 2,400 Kg. / M3) underneath Bed-Plate or Base Plate should be between 4 to 10 times the mass of the largest rotary machine under testing. Higher the mass better it is. The preferred and economical grade of cement concrete is M20 (1:1.5:3 mixes). However, cement concrete foundation mass depends upon soil conditions prevailing at the site of testing rigs. It is sensible to design a foundation rather than use the rule of thumb.

A well designed and well executed cement concrete mass foundation for the testing rig can be termed as FOUNDATION FOR QUALITY OF ROTARY MACHINES. Therefore, it is very important to design and construct compatible foundation for dynamic equipment like rotary machines. For proper design of cement concrete mass foundation for dynamic equipment some of the basic design parameters are required. These have been listed in page 4 of 4 here below.

Installation and alignment of mechanical equipment is not a trivial undertaking.

It is vital activity in design of the system as a whole and requires careful consideration in selecting the equipment and the methods employed for installation and alignment of the system to increase the economic importance of high reliability. The alignment tolerance values for particular parameter must be well defined right at the initial stage of system design.

Installation and alignment has direct bearing on efficient, economic, reliable, productive and trouble free functioning of the system for its useful life span. That said, it is simpler to undertake this important activity beforehand rather than waiting for the equivalent or alternative installation and alignment methods sought at the time of installing a system. These last moment installation and alignment activities inhabit lots of short cuts thereby defeating basic purpose of the system though it may appear to be working alright but only in short runs. Ideally, any system will perform to its peak efficiency only when alignment of the system is orthogonal with respect to the reference line or the reference plane or surface.

FOUNDATION FOR DYNAMIC EQUIPMENT / TESTING RIG

Every testing rig for testing of rotary machines, reciprocating machines, impulsive machines, impact type machines, or servo hydraulics system must have compatible civil foundation. Foundation for a testing rig must be very carefully designed so that the testing system is compatible with the overall testing and analyses requirements. For designing civil foundation for a testing rig following technical parameters for dynamic analysis and design of civil foundation are essentially required from the Buyer.

Therefore, is essential to invest some time in exploring these details and provide relevant technical parameters to the manufacturer / supplier for the equipment applicable to you.

1.Type of testing rig:

(a) Rotary Machine
(b) Reciprocating Machine
(c) Impact Machine / Impulsive Machines / Servo hydraulics

2.Type of equipment / machinery under testing:

(a)Independent
(b)Coupled in-line

3.Weights:

(a) Total weight of equipment
(b) Weight of moving machinery / components

4.Speeds / Strokes:

(a) Normal operating speed range
(b) Critical speed range
(c) Numbers of strokes / minute (Impact machine / servo hydraulic)

5.Dimensions and drawings:

(a)General arrangement drawing, plans and elevations
(b)Foundation outline drawing, plan and elevations
(c)Equipment dimensions
(d)Center of gravity locations of the equipment in X, Y and Z directions
(e)Foundation edge distance requirements
(f)Piping or electrical conduit requirements under the base plate

6.Unbalanced dynamic forces: Magnitude of the balanced vertical, lateral, and longitudinal pseudo-dynamic forces and torques transmitted to the test rig foundation during:

(a) Normal operating speeds
(b) At highest vibration at critical speeds
(c) At vibration level just short of tipping on high vibration and
(d)Under the maximum level of upset condition

7.Vibration limits: Maximum acceptable vertical and horizontal amplitudes imposed on the test rig foundation.

8.Soil Condition at the place (or nearby) of installation of testing rig:

It is established fact that the soil properties significantly influence the dynamic response of the testing rig foundation and the system thereon. Hence, it is very important to have the following technical parameters of the soil for foundation design for a testing rig.

(In most of the plants the details of soil conditions are always available with Civil Engineering and Design Department. If not then soil investigation for following technical parameters will have to be done and furnished for design of a test rig foundation.)

Soil mass density
Bearing capacity of soil
Young’s Modulus of Elasticity
Shear Modulus
Poisson’s Ratio
Soil damping
Coefficients of uniform compression of the soil
Coefficients of non-uniform compression of the soil
Coefficients of uniform shear of the soil
Coefficients of non-uniform shear of the soil

Hope the above information will benefit you in arriving at right selection of the system. We have endeavored to present here above our experiences through exposure to the industry. If you have any comments or suggestions, we will welcome it.

We will also welcome your queries, if any.