Page 294 - Pipeline Rules of Thumb Handbook

Gas—Compression 281

Table 2
Approximate Mechanical Losses as a Percentage of Gas Power Requirement*

Gas power Requirement
Mechanical
English Metric Losses, L m
(hp) (kW) (%)

0–3,000 0–2,500 3
3,000–6,000 2,500–5,000 2.5
6,000–10,000 5,000–7,500 2
10,000+ 7,500+ 1.5

* There is no way to estimate mechanical losses from gas power requirements. This table
will, however, ensure that mechanical losses are considered and yield useful values for estimat-
ing purposes.

(
Determine polytropic head, H p : Mechanical losses L m ) = 25.% ( from Table 2)
L m = (0 025 )(4 290, )
.
(
1
H p = Z RT n n - )[ r p ( n- ) 1 n - ] 1
1
a
= 107 hp
[
.
= (0 94. )(1 545 45 5, . )(520 )(6 88. ) (3 33. ) 16 88 - ] 1
= 21 800, ft lbf lbm
-
PWR s = PWR g + L m
Determine the required number of compressor stages, q: = 4,290 + 107
= 4,397hp
q = ( [ 26 1 . MW ) ( 1 k Z T )] 05 .
1 1
.
= ( [ 26 1 . )(45 5 . ) (1 126 )(0 955 )(520 )] 05 .
.
Determine the actual discharge temperature:
.
= 146
max H p/ stage from Figure 1 using q = 1.46 ( n 1) n
-
T 2 = T r p1 ()
.
Number of stages = H p max. H stage = 520 3 33) 16 88
( .
p
= 21 ,800 9 ,700 = 619∞ R
= .225 = 159∞ F
= 3 stages
Determine the required rotational speed: The discharge temperature calculated in the last step is the
same as that calculated earlier only because of the decision to
N N nom[ H H pnom ¥ no stages] . 05 use the inlet k-value instead of the average k-value. Had the
=
.
p
)( )]
5
= ,900 [ ,21 800 ( ,10 000 3 . 05 average k-value been used, the actual discharge temperature
would have been lower.
5
= ,030 rpm
Determine the required shaft power:
Source
PWR g = mH p 33 000 n p
,
,
= (5 000 )(21 800, ) (33 000, )(0 77. ) Estimating Centrifugal Compressor Performance, Gulf Pub-
= 4 290 hp lishing Company, Houston, Texas.
,