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.

  1. The basic concept of surge point

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.

  1. The basic method of drawing 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:

  • The ratio of compressor outlet to inlet pressure, that is, Pd/Ps (Y-axis) and the percentage of the design pressure of the orifice differential pressure h to the inlet pressure Ps, that is, h/Ps% (X-axis). See <Figure 1>.

 

 

 

  • The basic idea of the surge line calculation method introduced by TRICONEX is basically the same as the above method. The difference is that when the gas molecular weight MW changes, the above straight line is a broken line. See Figure 3.

 

 

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.

 

  1. Calculation of 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. 

  1. Conclusion

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.