Rod Loading of Reciprocating Compressors J

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International Compressor Engineering Conference School of Mechanical Engineering

1978 Rod Loading of Reciprocating Compressors J. D. Mowery

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Mowery, J. D., "Rod Loading of Reciprocating Compressors" (1978). International Compressor Engineering Conference. Paper 249. https://docs.lib.purdue.edu/icec/249

This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/ Herrick/Events/orderlit.html ROD LOADING OF RECIPROCATING COMPRESSORS

John D. Mowery, Manager of Engineering Cooper Energy Services Superior Operations Springfield, Ohio

1. GENERAL with the square of the compressor rotative speed. Figure 2 illustrates that the inertial load will also Rod loading is a major consideration for the designers vary with the reciprocating weight. One should also and operators of reciprocating compressors such as the notice that the inertial load is a "reversing" load one illustrated in the cross-section. One has to ascer in that it changes from tension to compression (and tain that the net rod load does not exceed the design back to tension again) during one complete rotation limitations placed upon the machine, which for the of the crankshaft. Superior compressors are shown in Table 1. These limit ations are determined by consideration of many stresses: b. Gas Load the rod compressive and tensile stresses, the rod column stresses, crosshead compressive and tensile stresses, the Figure 3 shows the gas load developed in a double rod and crosshead thread stresses, and the crosshead pin acting 12.5" diameter 6" stroke cylinder operating (wrist pin) shear and bearing (bushing) stresses. These from 100 psig suction pressure to 250 psig discharge stresses are evaluated for their strength of material in pressure. The gas load on each end of the piston is tegrity including the stress at crosshead pin bushing. determined by finding the pressure inside each end of The crosshead pin bushing stress must be considered not the cylinder at various points in the stroke. This can only for its magnitude, but also, and more importantly, be accomplished by approximating the compression for its direction of application. Since the crosshead pin process as an adiabatic process and using the relation bushing stress (or load) is many times the limiting para ship P2 = P] (V]/V2)k. This pressure is then multi meter in reciprocating compressor applications, we plied by the respective piston areas to give the head should take a closer look at its development, its effects, end and crank end gas loads. The total gas load is and its control. then found by adding the H. E. gas load and C. E. gas load. Figure 3 also indicates that the total gas load 2. CROSSHEAD PIN BUSHING LOAD (ROD LOAD) for a double acting cylinder contains a reversal. DEVELOPMENT c. Net Rod Load The load which is applied to the crosshead pin bushing is developed from two sources: the forces of inertia of (1) The net load applied to the crossheod pin bush the reciprocating piston, rod, and crosshead assembly, ing is found by the algebraic summation of the in and the forces resulting from compression of the gas in ertial load and the total gas load. Figure 4 illus the cylinder. Let us consider each of these forces indi trates the net rod load developed by the double vidually, and then combine them to give us the total acting 12.5" cylinder as well as the inertial and rod load. gas loads which comprise this rod load. Notice in Figure 4 that a load reversal of some 160° dura a. Inertial Load tion exists in every crankshaft revolution.

The inertial load is that force which develops as a (2) Net rod loads for single acting cylinders ore result of the weight (mass) of the piston, rod, and determined by the same procedure as for double crosshead assembly (including piston rings, nuts, acting cylinders. Figures 5 and 6 show the 12.5" crosshead pin, and balance weights) being in recip cylinder in single acting head-end and single rocating motion. Figure 1 illustrates the inertial acting crank-end applications respectively. Both force developed by a 12. 5" diameter 6" stroke single acting applications have opened the inopera cylinder with 250 pounds of reciprocating weight. tive end to suction pressure. As can be seen in This example also shows that the inertial load changes Figures 5 and 6, single acting cylinders have load

73 reversals which are both shorter in duration and inertial load and the gas load, We certainly must eval smaller in magnitude than in the majority of double uate how changes in each effect the rod load. acting cylinders. Notice that the reduction in reversal applies to both the total gas load as well b. Inertial Load Changes - Examination of Figures 7 as the net rod load. In fact, it can be seen that and 8 will reveal that by increasing the inertial force, if the compression ratio were higher, a reversal the net r,od load reversal also increoses, both in magni

for the gas load would not even occur 1 a very tude and duration. By analyzing the inertiol load common phenomenon for single acting applications. formula shown in Figure 1, one con see that the inertial load can be increased without a mojor design change in 3w. ROD LOAD EFFECTS the equipment by increasing either the rotative speed or the reciprocating weight. This gives us our first a. As mentioned previously, we must not only con two means of contrail ing the rod lood: sider the mognitude of the rod lood for design integ rity, but we must also observe its direction of oppli (1) Increasing the rotative speed of the compressor cation (i.e. tension or compression). The direction will increase the inertial lood ond the amount of of the rod load has a poramount effect on the lubrica the reversal. (See Figure 1) tion of the crosshead pin bushing. When the load is being applied to one side of the bushing, some finite (2) Adding weight to the crosshead and piston amount of clearance develops on the opposite side. assembly will also increase the inertial load and. This clearance, illustroted in Sketch 1, is filled with the size of the rod load reversal, {See Figure 2). oil thereby lubricoting and cooling that side of the bushing, In order to lubricote the other side of the Figures 7 ond 8 show the effects of inertial lood bushing, o clearonce must develop there also. A changes on the net rod load, reversal in the direction of application of the lood must occur for this to happen, And it is also evident c. Gos Lood Chonges- The effect of the total-gas load !hot the magnitude and duration of this reversal must on the net rod lood is quite different from the inertial be such that a complete filling of this clearance effect. Increasing the total gas lood may either in space with oil can be effected, This is necessary to crease or decrease the rod lood reversal.. In a single ochieve odequote lubrication and cooling. acting opplicotion, for instance, increasing the gas load on the operating end will decrease the reversal. b. Reciprocating compressors operating with non This can be seen by reviewing Figures 5 ond 6. On reversing loads are highly subject to bushing domoge. the other hand, however, increasing the gas load on Many post instances hove shown that a bushing con the inoperative end of a single acting cylinder (i.e. foil within a very few minutes while operating under opening the crank end of a small diameter cylinder to non-reversing loods, The foiled bushing, such os the discharge pressure instead of suction pressure) would one shown in the Photograph No. 1 & l~ wi'tl exhibit increase the reversal. Ample precautions must, there severe wiping scars and scratches over approximately fore, be employed in making changes to the gas load. 120° of its I . D. and moy very possibly show discolor The changes we can make to the gas load are categor ation from overheating. This damage, which is ized into cylinder configuration changes and operating characteristic of lubrication absence, will appear in condition chonges. the direction of the applied non-reversing load, or in the direction of the dominant load if the reversal is (1) Cylinder Configuration Changes marginal. (a) The end selected for operation (in single c. The Superior crossheod pin bushing design, shown acting applications) wi II effect the rod load in Photograph No, 2, has incorporated helicol oil reversal, Operating the crank end, which grooves on the I. D. to aid the lubrication. While has a smaller piston area, will produce a this design configuration reflects some performance smaller gas load and increase the reversal. improvements in non-reversing or marginal reversing lood applications, it does not provide a suitable non (b) Operating the cylinder in a double acting reversing load design thot could be used for non configuration will obviously increase the gas reversing condition. We must, therefore, continue to load and the reversal. analyze each job to control the rod lood in order that the non-reversing condition con be avoided, Let us {c) The size of the cylinder bore wi II effect look, then, at woys of controlling the rod load, the reversal in either way. Smaller bores in double acting cylinders tend to decrease the 4. ROD LOAD CONTROL reversal, but decreasing the bare in single acting configurations tends to increase the re- • a. The net rod load con be controlled by altering the versa I. constituents which comprise the rod load - namely the

74 (d) Decreased cylinder clearance will in f. High cylinder pressures are a natural for non crease the volumetric efficiency and increase reversing rod loads. They usually mean high gas the gas load. In single acting situations, loads, small cylinder bores, and sometimes single increased gas load will reduce the reversal; acting operation- all of which are susceptible to and in small diameter double acting cylinders, non-reverse Is. the increased gas load will expand the rever sal. 6. CALCULATIONS

(2) Operating Condition Changes Superior engineers have developed two Mark IT Fortran programs which are used to evaluate rod loads. One Reducing the compression ratio wi II decrease the program, which is primarily used to size compressor gas load and usually improves the rod load rever cylinders for specific applications, calculates the maxi sal. This can be accomplished by either lowering mum compressive and tensile rod loads using the operating discharge pressure or increasing the suction pres pressures external to the cylinder (i.e., external rod load). sure. One must be careful in lowering the dis The external rod load is a close approximation of the charge pressure by itself, however, since this actual rod load and indicates whether a reversal exists change can effect the reversal in either direction. and if so its approximate magnitude and direction. By defining a limit for the external rod load, one can also 5. GUIDELINES determine if the magnitude of the rod load exceeds the design loading capability of the machine. This limit is We are now fully aware of the necessity of avoiding properly set somewhat below the actual or internal rod non-reversing rod loads. And we have illustrated load limit (i.e., rod load based on pressures internal to various ways of manipulating the loads to accomplish the cylinder) since pressure losses occur through the valves the needed reversal. We must now apply this knowledge thereby producing a larger ratio inside the cylinder and to help us pinpoint non-reversing possibilities. Realizing hence a larger internal rod load. The use of an external the principles discussed above, we should be alerted for rod load limit not only benefits the engineers at Cooper a non-reversing rod load when an application contains Energy Services to quickly evaluate rod loads, but it also one of the following conditions: enables customers to periodically examine their rod loads as operating conditions change. a. Slow Speed Operation The second program developed at Cooper Energy Services ?low speed operation by itself is not necessarily a is used to calculate the internal rod load (both magnitude problem. But with other conditions present, slow and direction) for any cylinder at any angle of crankshaft speed could be a significant contributor to a non revolution from 0° through 360°. The program considers reversing rod load. all of the necessary parameters which can effect the rod load including cylinder configuration (piston and rod b. Single Acting Operation diameters, stroke, rpm, reciprocating weight, clearances, valve losses, single or double acting, tandem and tai I Non-reversing rod loads occur in single acting rod design, etc.), gas composition and compressibility, operation more than in any other situation. And and all operating conditions (suction pressure and tempera single acting head end operation (SAHE) is always ture, discharge pressure and temperature, barometric more susceptible to non-reversals than single acting temperature, etc.). The product of this program is a crank end operation (SACE). print out, giving for each specified angle of crankshaft revolution, the HE and CE gas loads, the total gas load, c. Small bore sizes in double acting cylinders the inertial load, and the net resultant of these compon approach a single acting condition and are non ents- the actual rod load. With this data readily avail reverse I prone. able, the acceptability of the rod load's magnitude andre versal can easily be analyzed. d. Low volumetric efficiencies (VE) often produce non-reversals. Low VE's result from high clearances, 7. SUMMARY particularly in unloading sequences where clearance is deliberately added. When performing unloading, The purpose of this discussion was to provide a thorough one should always remember that SAHE is more sus understanding of rod loads and their effects. While gas ceptible to non-reversals than SACE. The head end loads and external rod loads can be used as a general pockets should be opened first to avoid the non guide, they alone do not completely define the actual reversal. rod load. Each job must be analyzed to control the rod load within the machine capabilities such that a satis e. High compression ratios are apt to produce non bctory reversal is achieved. The tools provided here reversals. will help you to recognize the non-reversing possibilities and how to avoid their occurrence.

75 TABLE!

SUPERIOR COMPRESSORS

ROD LOAD LIMITS

INTERNAL ROD LOAD (LBS.) EXTERNAL ROD LOAD (LBS.) COMPRESSOR DOUBLE SINGLE DOUBLE SINGLE MODEL ACTING ACTING ACTING ACTING

W6 35,000 35,000 30,000

MW6 40,000 40,000 35,000

,_ ,_

~ATI-

IRTIOfl IRTIOfl

CYJ.!M)ER CYJ.!M)ER

Rill~ Rill~

~11'011 ~11'011

l.!!E l.!!E

WATE!I WATE!I

.JACRII::T .JACRII::T

I I

PIATOII PIATOII

LOW~ LOW~

~ATE ~ATE

lfALYI: lfALYI:

YALYII: YALYII:

U1'AINDI U1'AINDI

-U -U

,__ ,__

CNI:CJI CNI:CJI

YALYE YALYE

~IN ~IN

UOSIJCAD UOSIJCAD

SECTION SECTION

CROSS CROSS

COMPR~SSOR COMPR~SSOR

~A~ ~A~

-A1'IIVI -A1'IIVI

SUP~RIOR SUP~RIOR

TRANSVERSE TRANSVERSE

IAR IAR

I"'~CR I"'~CR

~ ~

~-lAD ~-lAD

uo uo

C'fLti!ER C'fLti!ER

Me~ Me~

TYACAL TYACAL

LINU LINU

fR£SSl,A£ fR£SSl,A£

~·~-R ~·~-R

HIGH HIGH

•LA-

-tiDII -tiDII

" " " " C> z 1- u· w z zoo 01 uO:

z I. I. Vl l/1 I --:J -----..-4. -:::>1-.-,.----1 u om· 1- :0~ w

0 <( 0 ..J

RPJ:•i RPJ:•i

RP!-1 RP!-1

500 500

00 00

,3600 ,3600

LOADS LOADS

-900 -900

-----

. .

"'-INERTIAL "'-INERTIAL 330 330

I I

,)00 ,)00

--l --l

I I

Q~ Q~

SIN SIN

- 2

(1 (1

~L ~L

/ /

60° 60°

J._ J._

JOQO JOQO

+ +

Q Q

HEDC) HEDC)

LOADS LOADS

(IN) (IN)

[cos [cos

(I.BH) (I.BH)

= =

2 2

(oo (oo

LENGTH LENGTH

\•!EIGHT \•!EIGHT

ANGLE ANGLE

£ £

900 900

.i.__ .i.__

270° 270°

STROl'.E STROl'.E

FIGURE FIGURE

11 11

£ £

DEGREES DEGREES

CRANK CRANK

IN IN

NERTIAL NERTIAL

ROD ROD

! !

10-5(WT)(ST)(RM}

l'l'PM l'l'PM

x x

(IN) (IN)

ANGLE ANGLE

RECIPROCATING RECIPROCATING

snn snn

DIAHETER, DIAHETER,

11 11

LBH LBH

1.4208 1.4208

CON~illCTING CON~illCTING

TOTAL TOTAL IN, IN,

CRANK CRANK

STROKE STROKE

RPU RPU

2400 2400

120° 120°

_L_ _L_

= =

6 6 250 250

900 900

14.5 14.5

12,5

= =

= = = =

= =

Q Q

RH RH

ST ST

WT WT

RL RL

LOAD LOAD

RL: RL:

ru~: ru~: ST: ST:

''lT: ''lT:

CYLINDETl.: CYLINDETl.:

\VHERE: \VHERE:

INE!l.TIAL INE!l.TIAL

2100 2100

___L__ ___L__

150° 150°

-35.--~ -35.--~

-30 -30 -25 -25

~ ~ {I) {I)

0 0

~15 ~15

~-20 ~-20 0 0

{I) {I)

•n •n

u u

-...] -...] 1.0 1.0 LBl·I RPH

RPI·f

900

300 250LBN

.3600

J60°

LOADS

INERTIAL

,...

30°

300 JJOO

JJOO

,....'\..

SIN

- 2

60°

J00°

JOQO

ANGLE

@os

(IN)

HEDC)

(Lm!)

= FIGURE

CRANK

LOADS (0°

LENGTH

900 / '7

900

WEIGHT

2700

270°

STROKE

,/'

DEGREES

ROD

1o-5(WT)(ST)(ruo~)

LBM

(IN)

RECIPROCATING

NERTIAL

ANGLE

DIAMETER,

300

IN.

1200 240°

120° 240°

RPM

1.4208

TOTAL STROKE CONNECTING

IN.

CRANK

= = 900 =RPM 12.5

= 250 14.5

WT Q RM RL

LOAD

ruo~: WT:

RL: ST:

-----

_...... -

WHERE:

CYLINDER:

.--

INERTIAL

150°

2~00

150° 210°

------

-.35

s:: 0 §

•.-I

E-< ><

.-1

~ ;_i! ~ Q

~ s

......

C"''

OJ 0 PSIG

.360°

250

STROKE

PDisch,

CYLINDER,

LOAD PSIG

100 GAS

DIAUETER

LOAD

TOTAL

GAS 12.5 PSuct.

.3000

-00

'c=;.

•

LOADS

ANGLE

90°

270°

90°

2700

GAS

FIGUnE

CRANK

ACTING

2400

120°

DOUBLE

150° 210°

150° 2100

1800

CEI

-2

-3

~-15

.-~ 0

~-2

In !II

r-1

~ . ~ s

,.....

CXI

1-4 LOAD

LOAD

PSIG

GAS

ROD

250

.360°

STROKE

TOTAL

....

PDisch,

',,

~::_-NET

'"'

CYLINDER,

PSIG

330°

3)0:0

100

·-=-:-A;po

DIAMETER

ACTING

12,.5' PSuct.

---

cow

300° JQOO

LOAD

DOUBLE

'..../

AUGLE

GAS

FIGURE

CRANK

900 270°

,.,..__

ROD

120° 240° 240° 120°

-----

'... 6

150°

:no

......

------

-351

Q) !/) ,:::

§201---

•n

>< E-<15

~· ~ ~CE 1'""'1

8 o-5 iDC

(X) N LOAD

PSIG

GAS

250

HEDC

J600

J60

STROKE

'-~---NET

......

PDisch.

......

CYLINDER

'::::--

CYLINDER,

......

PSIG

......

JOO

.....

JJOO

......

100

DIAHETER

-~oo

AC·TING

PRESSURE

12.5

PSuct.

....-;

~ ---

60°

J00°

600

/-'.

SINGLE

/-'.

5 _/

....

~7"/

ANGLE

SUCTION

..-:;

FIGUilE

CRANK

LOAD

270

GAS

OPEN

ROD

120° ~oo

END

CRANK

150° 2100

--·-

I r

! I i

·5~

15: 20

1ol-

-5~

-25•-

-JO

(()

§ a

-J51e

E-< ·~

~DC ~

.-I

~ 8 3

......

-cEb

§-15l-

~-20 ~· a! 0 <'6 (II

CD w PSIG

LOAD

INERTIAL

ioAi:l

3600

250

GAS

STROkE

PDisch,

CYLINDER,

PSIG

PRESSURE

30°

330°

CYLINDER

100

DIAMETER

12.5 PSuct,

ACTING

SUCTION

60°

JQOO

LOAD

GAS

ANGLE

FIGURE

SINGLE

CRANK

900

270°

"<:""

....

OPEN

--...

.,-

......

END

......

_...

120°

2AQ

HEAD

ROD

_.--..,

...,..-

..-..-

---

150° 21d

-15 -25

-J518Qo

0 8' §

~-20 lll

•n

11>0

CP ,

RPH RPJ.i

LOAD

900

500

ROD oo

ROD

360°

SAHE

SAHE NET

NET

--'-

CHANGE

------

]

30°

330°

___

CYLINDER

-:7

INTERNAL

------

6o0

3000

60°

300°

ACTING

PRESS;

ANGLE

---

_.,/

FIGURE

CRANK

::::....

SINGLE

......

[Do

C)(

9QO '1~---/

270°

......

EF"FECTS

......

;;;:;"}_,..

SUCTION

---

LOADS

_____

120° 24QU

ROD

OPEN

----

_:__::>---

---~~;Tno~ -

NET

END

-=-"";::-~---

1~0o

210°

1500 2100

-=----=----

CRANK

---

- ,

r--~~-

·-

180°

25 30

-5

.-~20

-10

-15

!/) i)

g. -30

(/) m20 t

•

E-o

•.-I

~25

!;i! S

g ..

~eEl

~1+

V1 co

! !

00 00

RPH RPH

oo oo

3600 3600

LOAD LOAD

900 900

. .

RPU RPU

ROD ROD

LOAD LOAD

500 500

SAGE SAGE

NET NET

. .

CHANGE CHANGE

ROD ROD

I I

SACE SACE

)j0° )j0°

30° 30°

330° 330°

~ ~

--r;ET --r;ET

-~ -~

--.--;::;,:;::::;..----

INTERNAL INTERNAL

.. ..

0 0

I I

·. ·.

609.---

CYLIND~R CYLIND~R

300° 300°

PRESS PRESS

8 8

ANGLE ANGLE

ACTING ACTING

~ ~

[0° [0°

90° 90°

270° 270°

~--:;,/ ~--:;,/

~ ~ ......

FIGURE FIGURE

EFFECTS EFFECTS

CRM!K CRM!K

SUCTION SUCTION

SINGLE SINGLE

TO TO

I I

240° 240°

120° 120°

240° 240°

120° 120°

...... ------

---

------::...---

-...... -......

LOADS LOADS

...... _ ...... _

OPEN OPEN

' '

....,., ....,.,

ROD ROD

'b' 'b'

~ ~

210° 210°

1j0° 1j0°

END END

1$0 1$0

", ",

......

NET NET

HEAD HEAD

-....-----~-

1 1

180° 180°

I I

5 5

-5 -5

DC DC

CE CE

10 10

15 15

20 20

25 25

30 30

-35 -35

-30 -30

-25 -25

-15 -15

-10 -10

!I) !I) 0 0

m-20 m-20

i i

!': !':

·.-I ·.-I

!ll !ll

~ ~

g g

K K

~ ~

Q Q

s s

~ ~

s-t s-t

::1 ::1 ......

~ ~

r-t r-t

......

c2:' c2:'

0\ 0\ OJ OJ PHOTOGRAPH NO. 1

87 PHOTOGRAPH NO, lA

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