Occasional Load

Occasional loads B31.3

1. Occasional Loads In Piping
2. Occasional Load Meaning
3. Occasional Loading
4. Occasional Load

Occasional Load Factor, K The User can enter a value for the occasional load factor, if desired. A value of 1.15 should be entered for occasional loads acting less than 10 percent of the operating period and a value of 1.2 should be entered for loads acting for less than one percent of the operating period. Occasional loads. These type of loads are imposed on piping by occasional events like wind, earthquake etc. Wind loads are considered for lines with external diameter 14” NB (including insulation) or above and at elevation 10 meters or higher from the ground level.

Occasional loads B31.3

Understanding piping loads on rotating equipment including harmonic vibrations is crucial for a successful piping system design. AutoPIPE, Bentley’s premier pipe stress and analysis software application, helps engineers overcome the challenges in analyzing loads on rotating equipment. In this paper we will review piping loads on API 610 Pumps, NEMA SM 23 Steam Turbines, API 617 Compressors, user equipment from vendors using reference points, and the vibrations from those pieces of rotating equipment.

API 610 Pumps

Piping systems often include pumps, which are usually massive enough to be modeled as a rigid anchor at the inlet (or suction) and at the outlet (or discharge) points. However, API 610 Standard establishes limits in Table 5 on the magnitude of external forces and moments by nominal pipe size by which are acting at the pump nozzles as a result of the forces generated by the piping system.

Below is Table 1 from API 610 which identifies the different types of pumps covered by the code. There are many different variations of horizontal and vertical pumps, and AutoPIPE streamlines efficiency by narrowing it down to these two selections.

AutoPIPE allows the definition of a variety of pump arrangements by specifying defined piping points which correspond to the nozzle connection points (e.g. the inlet and outlet). Based on the code guidelines, the external forces and moments calculated for each defined load combination at each nozzle can be checked in order to determine that the nozzle loadings are within the code allowed limits.

Checking Loads on Cold Equipment

In this model shown in figure 3 which can be found in the AutoPIPE Help under Modeling Approaches, a horizontal pump has been built. Modeling the pump with disconnected piping and no piping to represent the pump is sufficient for “Cold Equipment.” First, the piping system is defined. A different segment is used for the suction which is Segment A and discharge which is Segment B lines attached to the pump. These suction and discharge points can either be on the top, end, or side of the pump with respect to the pump shaft.

These lines can be modeled at the correct coordinates in physical space or at any position and use the “Override nozzle coordinates” which is adequate for cold systems but not for hot systems. The suction and discharge lines will have operating temperatures and pressures which relate to the specific line. After an analysis is performed on the system, the Tools Rotating Equipment command can be used to specify the piping points which connect to the pump nozzles, and the location of the pump center relative to one of the nozzle points, for vertical pumps, the center point location is not required. When an equipment report is generated, AutoPIPE checks the external forces acting on the pump nozzles according to API 610. API 610 defines the center point of the pump as the point on the shaft midway between the pedestals. The center point doesn’t necessarily need to be a node within your piping model, and that is why there is a reference point with offsets listed for the center point. The other critical points are the suction and discharge points which need to be labeled as top, end, or side in respect to the pump shaft.

Checking Loads on Hot Equipment

So, we have mentioned cold equipment but how do we check the pipe loads on hot equipment? In this example we have a hot pump modeled using weightless RIGID pipe, which is purple, with no pressure and heats up and expands.

With the pump modeled follow the same method for the critical points as you did with the cold pump in the rotating pump dialog and pump report generation. As mentioned before, there are two main configurations of pumps to API 610, horizontal and vertical. Figures 23, 24, & 25 show example configurations of horizontal pumps for the different types.

As you may recall Table 5 gave the allowable loads for API 610 pumps. But there are also acceptable loads above that in Table 5 as long as the conditions specified in F.1.2 a through c are satisfied. Condition A says that none of the individual component forces and moments acting on each nozzle shall exceed twice the value listed in Table 5. Condition B states that the resultant forces and moments acting on each nozzle shall satisfy the equations shown.

The final condition, C, states that the resultant force and moments also have to meet the requirements of the equations shown. Now there was a change between the 9th edition to the 10th edition.

The code added the absolute value operation on the left side of the equation. AutoPIPE includes the 8th and 11th edition of the code (which also includes the 10th edition changes).

Vertical pumps have examples shown in Figures 21 and 22 below.

As with the horizontal, there are also acceptable nozzle loads above that in Table 5 for vertical pumps as long as the principal stress does not exceed 5,950 psi.

The equations shown is how the code specifies how the principal stress is to be calculated. The side nozzles are also allowed to be double what is listed in Table 5, but only as long as the principal stress is below that 5,950 psi limit still.

Pump Module or Reference Point Example

Occasional Loads In Piping

You can also model pumps that are not designed to API 610 (e.g. there are ANSI or DIN pumps) using the user module in the rotating equipment dialog or with reference points. Here is an example of a hot pump being modeled using rigid pipe.

Occasional Load Meaning

Instead of using the API 610 module, the user modeled the pump with the user option. The special manufacturer had its own allowable loads which are defined in the User Rotating equipment dialog. We can use either Reference points or the User equipment to check the pump loads. Reference Points are defined at the Suction & Discharge nozzles. The rotating equipment report checks all load combinations and reports nozzle loads which fail in the operating case and to use the reference points.

You can put in the allowable forces and moments in either the global or local coordinate system. Then you can run the model analysis and create a reference output report. The solution in this case was to install a pressure balanced bellows on the suction nozzle. This reduces the nozzle moment by a factor of 100 from 250,000 down to 2,300 foot pounds.

Steam Turbines Designed to NEMA SM 23

Now that we have that covered, let’s talk about steam turbines which are designed to NEMA SM 23. The modeling of turbines is not usually required because they are massive enough to be considered rigid when compared to the piping. However, there are limits on the magnitude of external forces and moments which can be acting on the equipment nozzles as a result of the forces generated by the piping system. Thus, codes have been established, NEMA SM 23, which provides guidelines for ensuring that these limits are not exceeded.

In this model below, a two-stage turbine is to be included in the piping system. First, the piping system is defined. A different segment is used for each pipe line that connects to the turbine. Each line will have operating temperatures and pressures which relate to the specific line. Next, an analysis is performed on the system. Once the analysis is completed, you can specify the piping points which connect to the turbine nozzles.

When the equipment batch report is requested, AutoPIPE will check the external forces acting on the turbine nozzles according to NEMA SM 23-1991. That batch report checks the nozzle loads to the equation below with the force and moment resultants. If the nozzle nominal pipe size is larger than 8 inches, De will need to be calculated as shown. The 2nd limit for the nozzles states that the combination of each resultant load on each nozzle must meet the equation shown.

Dc is defined as a circular opening equal to the total areas of the inlet, extraction, and exhaust openings up to 9 inches. If Dc would be larger than 9 inches, then Dc shall be calculated as the equation shown.

One final limit for turbines is that each nozzle load shall not be larger than each of the equations shown. Figure 8-15 just shows the orientation that should be used for turbines.

Compressors Designed to API 617

The modeling of compressors is not usually required because they are massive enough to be considered rigid as well when compared to the piping. However, there are limits on the magnitude of external forces and moments which can be acting on the equipment nozzles as a result of the forces generated by the piping system. Thus, codes have been established (API Standard 617) which provides guidelines for ensuring that these limits are not exceeded.

In this model, a compressor is included in the piping system. First, the piping system is defined in the model. A different segment is used for each pipe line that connects to the compressor. Each line will have operating temperatures and pressures which relate to the specific line. Next, an analysis is performed on the system. When an equipment batch report is requested, AutoPIPE will check the external forces acting on the turbine nozzles according to API 617-1995.

The batch report checks the nozzle loads to the equation shown using the force and moment resultants. If the nozzle nominal pipe is larger than 8 inches, De will need to be calculated as shown which is similar to turbines.

The 2nd limit for nozzles states that the combination of each resultant load on each nozzle must meet the equation shown. Dc is defined as a circular opening equal to the total areas of the inlet, extraction, and exhaust openings up to 9 inches. If Dc would be larger than 9 inches, then Dc shall be calculated as the equation shown.

One final limit for compressors is that each nozzle load shall not be larger than each of the equations shown. Figure 2.E-1 just shows the orientation that should be used for compressors.

Harmonic Analysis

All of these pieces of equipment will cause vibrations in your piping system. These vibrations should be taken into account in order to avoid hitting a natural frequency of the piping system and cause a piping failure. Vibrations can come from a few different sources:

1. Mechanical from rotating equipment - These vibrations can be accounted for using a harmonic displacement analysis.
2. Flow induced vibrations can be caused from pulsation flow. Pulsation flow can be accounted for using a harmonic force analysis.
3. A pressure surge from a hydraulic hammer effect can also cause a vibration - These vibrations can be accounted for using force and displacement time history and the fluid transient module.

We will be covering the pulsation vibration with our first example. Complex vibrations can be decomposed into several simple harmonics which are out of phase. The response of each harmonic is obtained separately. The total response is calculated by combining the response of individual harmonics. It is important to consider at least the first 3 or 4 harmonics for reciprocating equipment.

Occasional Loading

The first thing we need to do is find the natural frequencies of the system. Natural frequency and mode shapes are properties of the structure and are dependent on the mass and elasticity. The natural frequency and mode shapes describe the tendency of the structure to vibrate when subjected to dynamic loading. The number of frequencies and mode shapes with which a structure can vibrate depends on the number of mass degrees of freedom in the structure. Dynamic loading is usually a combination of several frequencies and the response is therefore also a combination of several modes of vibration.

Dynamic loads tend to excite frequencies closer to their own frequency. We should look for mode shapes similar to the applied dynamic loading displacement shape. Generally, the first mode of vibration is the one of primary interest. The first mode usually has the largest contribution to the structure’s motion. So let’s take a look at an example.

Set the following parameters which are specified: The Mass Points per Span should be set to A with the Cutoff Frequency of 200 Hz. Entry of A, which stands for automatic, or a value larger, greater than 0 will allow mass discretization for dynamic analysis. AutoPIPE will then generate the specified number of mass points between existing piping points, thus refining the model for the purposes of dynamic analysis without having to manually enter all the required points.

The following options are available for input:

• If zero is entered, no mass point discretization will occur. This is the default. If a value greater than 0 is entered, AutoPIPE will generate that number of mass points spaced equally between piping points, valid range is 1 through 9.
• If A is entered, the number of mass points spaced equally between any two piping points will be generated automatically based on the dynamic properties of each pipe span and on the value entered in the Cutoff frequency field.

Next, a modal analysis should be run and a frequency report can then be created. As you can see we have only captured about 60% of the modal mass at the 200 Hz cutoff frequency. It is recommended that you have at least 75% of the modal mass captured. Thus we will need to run the modal analysis again with a higher cutoff frequency.

We have modified the cut off frequency to 1000 Hz now to get our minimum 75% modal mass capture. But now we have also changed our cutoff frequency we need to change it in the Edit Model Options as well.

It’s always a good idea to delete your old analysis results before beginning a new analysis. In the main menu, select Load, and Harmonic to get to the Harmonic Load dialog where we would select our RPM276 from the Harmonic Load Name pull down menu. The force load harmonics RPM276 are acoustic harmonics which are calculated by PULS and imported into AutoPIPE at the 2 locations A5 and A81.

Being Acoustic they are the induced vibration harmonics due to the gas pulsation flow from the compressor. Note: these harmonics can be applied at any point in the model; they don’t have to be a support point.

In the Dynamic Analysis dialog, select the Harmonic tab and we turn on our data set RPM276. We also suggest turning on the missing mass feature which will help capture all the modal mass up to 100%. This feature can hog up a lot of computing power so you don’t need to turn this on if you went through the procedure of capturing at least 75% of the modal mass like we did before. In the Confirm dialog, click yes to re-analyze and make sure you analyze your static modal in your harmonic. In the main menu, select Tools, Combinations or you can select the Load Combinations icon from the Main toolbar to review the load combinations for this model.

In the Load Combinations dialog, select the Code Combinations tab and notice that the Allowable Stress for the Sustain + Harmonic 1 and Sustain + Harmonic 2 is set to Automatic.

AutoPIPE does not have a cyclic fatigue type of stress and therefore we need to add a new code combination with just H1 by itself for the RPM of the harmonic from a rotation compressor to capture the fatigue allowable comparison. In the Load Combination dialog, select the Code Combination tab and add a new load case H1. Now, change that default Allowable Stress setting from Automatic which is typically 1.33 times the Stress allowable to 4,180 psi.

Why? Because to get the allowable stress for the created H1 load case, you need to review a fatigue curve. NUREG CR-6909 is the source of mine and then you find your allowable stress for the number of cycles that you plan to design for.

In this example we will design for 30 years and use 5 Hz as our first harmonic. Thus for 5 Hz which is cycles per second times 3600 seconds per hour times 24 hours a day times 365 days a year times 30 years we get 5E9 cycles. Looking at the tables and we interpolate between the values we find an allowable stress of 8.3 ksi. But since that is an amplitude we need to half that value to get our allowable stress for the H1 load case which is the 4.18 ksi we set before.

When the model is run again we see the maximum occasional stress ratio is 3.01 with the proper allowable.

Example: Mechanical or Shaking vibration from the compressor

We enter our 2 displacement harmonics received from the compressor manufacturer. Enter the compressor Nozzle rotational stiffness which is also from the manufacturer. To capture all the correct modal mass as before enter Mass Points equals A with a cutoff frequency of 50Hz, which is recommend to be at least 1.5 times the highest harmonic which was 19 Hz for this example.

The cut-off frequency is set to 50 Hz when we get into the dynamic analysis dialog. We will also select the RMS combination which is the most common method to combine Harmonics. ZPA is also checked to improve our nozzle reactions.

We can see some of the piping displacements in the Harmonic H1 load case to be 0.41 inches at the compressor and 2.84 inches at point A8 and 1.0 inch at B14 bend. So this results in high stress.

Occasional stress due to compressor “shaking” is 3 to 5 times the allowable stress. So we will need to re-design the system, perhaps introduce more flexibility into the system piping or add an expansion joint to isolate the vibration at the compressor. For example, reducing the anchor rotational stiffness to 2000 foot pounds per degree lowers the stress ratio to below 3 to 4 overstress.

Occasional Load

Here is one solution where we changed some guides, added a linestop and increased the tee thickness next to the compressor, so the resulting occasional stress was reduced to 81% of the design capacity.

AutoPIPE to model Rotating Equipment

In conclusion, AutoPIPE can be used to model many different types of rotating equipment like API 610 pumps, other types of pumps or equipment which have manufacturer allowable loads, turbines designed to NEMA SM 23, as well as compressors designed to API 617. These pieces of equipment can cause vibrations which AutoPIPE can also take into account to make sure your piping system won’t fail.

If you’d like to learn more about AutoPIPE check out our Bentley Communities website for user information and support or check out Bentley’s OnDemand webinars!