Discussion on Calculation Method of Surge Line of Rich Gas Compressor
1 Introduction
The catalytic cracking rich gas compressor is a kind of compressor, which is very representative. For this reason, the catalytic cracking rich gas compressor is taken as an example to discuss the calculation method of the surge line of the rich gas compressor control system.
At a certain speed, when the pressure ratio is constant, ensure that the compressor inlet flow rate is not lower than the value of the minimum flow control point, which is the surge point. For example: at a certain speed and pressure ratio, the surge point of the compressor inlet flow, the minimum flow value is 30% of the maximum flow, if the device processing capacity is reduced, the compressor inlet flow is less than 30%, that is, 25% In order to ensure the minimum flow value of the compressor inlet, it is necessary to use the compressor outlet volume to return to the inlet to supplement the insufficient 5% flow. This serves the purpose of anti-surge. Connecting the surge points into a line is called a surge line. For safety, the general surge line has a certain safety margin (7~10%), which is called the anti-surge line.
The basic principle of the compressor surge control system is shown in <Figure 2>. The pressure ratio (outlet pressure/inlet pressure) Pd/Ps and C* are used as the surge curve. Its basic shape is a parabola, and Pd/Ps and ( C*) 2 The surge line obtained when drawing is basically a straight line shape (after simplification, C2, where h is the differential pressure of the orifice plate, is a linear input).
Among them: Pd------outlet pressure, KPa;
Ps------Inlet pressure, KPa;
C ------constant, (determined by the size of the orifice), m2;
H ------The differential pressure of the orifice plate (the relationship with the flow rate is Q2=h), KPa;
Based on this method, there are basically two ways to calculate the compressor surge line:
In Figure 3, the A1 and A2 surge lines are the surge lines of two different molecular weights (14.0 and 24.2) of fixed molecular weight. A3 and A4 are the surge line conditions of variable molecular weight. TRICONEX company uses a variable molecular weight line composed of A3 and A4 as the surge line.
Method 1: Do not consider changes in molecular weight
The surge characteristic curve of a rich gas compressor can be regarded as a straight line near the operating point.
=V*+K
Among them: h ------- orifice inlet flow differential pressure value KPa.
Pd------outlet pressure (A) KPa;
PS------Inlet pressure (A) KPa;
V ------slope (ctgα);
K ------ constant (intercept);
Calculate the determination of compressor anti-surge line parameters:
Maximum design flow of orifice (standard) Q nmax=40000
Basic pressure of orifice inlet (absolute pressure A) Psb=101.3KPa
Orifice plate basic inlet temperature Tsb (°K=°C+273.15) Tsb =273.15°K
Orifice compression factor Zb Zb=1.0
Compressor inlet pressure (absolute pressure A) = 200.0KPa
Compressor inlet temperature Ts (℃) =40℃+273.15=313.15°K
Maximum working point molecular weight MWmax MWmax=32.82
Working point design molecular weight MWb MWb=28.841
Convert volume flow () to standard flow ():
=*==*1.722143
Surge point calculation table I: (molecular weight 28.814) take the middle three points
|
A1 |
A2(Working point) |
A3 |
(A)
|
1130.0(KPa) |
1390.0(KPa) |
1730.0(KPa) |
(A) |
200.0(KPa) |
200.0(KPa) |
200.0(KPa) |
= |
*100%=100% |
100% |
100% |
5.65 |
6.96 |
8.65 |
|
10700.0 |
12200.0 |
14000.0 |
|
=46.07% |
=52.5% |
=60.27% |
|
:% |
(46.07%)2=21.22% |
(52.5%)2=27.56% |
(60.27%)2=36.32% |
=21.22% |
27.56% |
36.32% |
according to =
∵
∴
Among them: --------The distance between the surge line and the anti-surge line is the percentage of the differential pressure range.
-------The distance between the set value of the regulator and the surge point is 7%~10% of the surge flow (generally 10%).
The relative flow percentage of surge point. Take any middle point
…
=(52.5%*10%)%* [2*52.5%+(52.5%*10%)%]
=5.25%*110.25%
=5.788%
Change 10% intercept: △K==0.05788
Intercept of surge line: b=0.3633-=-0.07237
To
Slope: V=ctgα===0.0517
Intercept of anti-surge line: K=b+△K =-0.07237+0.05788=-0.01449
Method 2: Consider the change of molecular weight
The surge characteristic curve of a rich gas compressor can be regarded as a broken line. The calculation of surge line takes the calculation method of TRICONEX surge line as an example:
① Calculation formula of pressure ratio (absolute pressure ratio): Y coordinate
== = 5.65
among them:
Pd ------------ Compressor outlet pressure KPa (A)
Y2 ---------- Compressor outlet pressure KPa (G)
Compressor inlet pressure KPa (A) Ps
Y3 ---------- Compressor inlet pressure KPa (G)
To
②. Maximum mass flow Mmax:
Mmax===58555.9 Kg/h
among them:
Max ---Maximum standard flow rate Nm3/h
Compressor design inlet pressure (absolute pressure A)
Maximum molecular weight
----------Unit constant
----------Compressor design inlet temperature (°K)
Compression coefficient of orifice plate Zb
Symbol and unit parameter comparison table
R |
ρ |
P |
T |
Z |
Unit constant |
density |
pressure |
temperature |
Compression factor |
10.73125 |
Lbs/Ft3 |
Psia |
°R=°F+459.67 |
Dimensionless |
0.084784 |
Kg/m3 |
Kg/cm2 |
°K=°C+273.15 |
Dimensionless |
8.3145 |
Kg/m3 |
|
|
|
③、Constant C:
C===90.4373
among them:
------Maximum mass flow Kg/h
------ Compressor inlet temperature (absolute pressure) KP
------Maximum molecular weight
Inlet temperature°
Compressor Compression Coefficient Zs
,,,
④ Density ρ:
Ρ===2.21332Kg/m3
among them:
Max---------molecular weight
Inlet pressure (absolute pressure) KPa
Unit constant
Entry temperature °K
Compressor Compression Coefficient Zs
To
⑤ Flow conversion ratio (X coordinate)
=%=%=18.6316%
among them:
-------Density Kg/m3
------ Volume flow m3/h (Mass flow M=* ,Kg/h)
Inlet temperature °K
Compressor compression factor Zs
---------Constant
Inlet pressure (A)KPa
Compressor molecular weight
Surge point calculation and preparation table II: (molecular weight 28.814) performance curve is slightly max=40000.0Nm3/h
|
A1 |
A2 |
A3 |
A4 |
(A)
|
1130.0(KPa) |
1390.0(KPa) |
1730.0(KPa) |
2170.0(KPa) |
(A) |
200.0(KPa) |
200.0(KPa) |
200.0(KPa) |
200.0(KPa) |
5.65 |
6.96 |
8.65 |
10.85 |
|
10700.0 |
12200.0 |
14000.0 |
17000.0 |
|
18.6316% |
24.2216% |
31.8963% |
47.0307% |
The same method is used to calculate the maximum molecular weight (MWmax=32.82) surge point calculation table III:
|
B1 |
B2 |
B3 |
B4 |
(A)
|
1250.0(KPa) |
1562.5(KPa) |
1968.75(KPa) |
2500.0(KPa) |
(A) |
200.0(KPa) |
200.0(KPa) |
200.0(KPa) |
200.0(KPa) |
6.25 |
7.8 |
9.84 |
12.5 |
|
11000.0 |
12500.0 |
15300.0 |
19000.0 |
|
22.4287% |
28.9627% |
43.3912% |
66.9154% |
Combine Table II and Table III to give a set of surge curves (five points) as broken lines, and make Table IV:
|
C1 |
C2 |
C3 |
C4 |
C5 |
(A)
|
1130.0(KPa) |
1562.5(KPa) |
1968.75(KPa) |
2170.0(KPa) |
2500.0(KPa) |
(A) |
200.0(KPa) |
200.0(KPa) |
200.0(KPa) |
200.0(KPa) |
200.0(KPa) |
5.65 |
7.8 |
9.84 |
10.85 |
12.5 |
|
10700.0 |
12500.0 |
15300.0 |
17000.0 |
19000.0 |
|
18.6316% |
28.9627% |
43.3912% |
47.0307% |
66.9154% |
Input the five surge points of the surge line into the surge function module provided by TRICONEX. After setting the parameters, the anti-surge line (generally 7%~10%) and the percentage of the anti-surge line downward movement are automatically generated (Generally 2%). In addition, there are some other functions: For example, the proportional function can ignore the adjustment of the controller and force the anti-surge valve to open. When the operating point suddenly moves to the anti-surge line, the anti-surge valve is opened by the surge circling function. , Quick opening and slow closing functions, and manual, semi-automatic, fully automatic switching operations and other functions.
The discussion of this anti-surge line is only one of the calculation methods of the surge line. For example, there are also: the ratio of the inlet and outlet differential pressure to the inlet pressure (Pd-Ps)/Ps surge line calculation method. This discussion on the calculation method of surge line is just to infer other things, the purpose is to let everyone master the calculation method of surge line. In addition, the temperature compensation of the surge line is more important, and reasonable temperature and pressure compensation can ensure the accuracy of the surge line. There are generally two methods for surge line temperature and pressure compensation.
1) The surge line at a certain inlet temperature, temperature and pressure compensation for the compressor inlet flow (applicable to rich gas compressors, etc.).
2) The compressor inlet flow rate is not compensated for temperature and pressure, and the compressor surge line is compensated for temperature and pressure (applicable to axial fans, etc.).
The temperature and pressure compensation content is relatively simple, so I won't discuss it here.
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