Method for pressure and temperature control of a fluid in a series of cryogenic compressors

Abstract

A method for pressure and temperature control of fluid in a series of cryogenic compressors. An actual speed for each compressor and an actual inlet pressure and actual inlet temperature at entry are determined. The maximum speed for each compressor and a desired inlet pressure for the first compressor is provided. A speed index for each compressor is determined from the maximum speed and actual speed of each compressor. A proportional value is determined from the deviation of the actual and desired inlet pressure. A priority value is determined from the smaller of the proportional value and the smallest speed index. A desired inlet temperature for the first compressor and a desired speed for each compressor are determined from the priority value. The actual inlet temperature is adjusted to the determined desired inlet temperature and the actual speed for each compressor is adjusted to the determined desired speed.

Claims

1. A method for pressure and temperature control of a fluid in a series of cryogenic compressors, said method comprising: detecting an actual speed for each compressor, detecting an actual inlet pressure and an actual inlet temperature at the entry of the most upstream, first compressor of the series, specifying a desired inlet pressure for said first compressor of the series, determining a speed index for each compressor from a maximum speed of the respective compressor and the actual speed of the respective compressor, determining a proportional value from the deviation of the actual inlet pressure from the desired inlet pressure, determining a priority value, wherein the priority value is determined from the proportional value, if the proportional value is smaller than the smallest speed index of all compressors of the series, and wherein the priority value is determined from the smallest speed index among all compressors of the series, if the proportional value is greater than the minimum speed index among all compressors of the series, determining a desired inlet temperature for the first compressor of the series and a desired speed for each compressor, with the aid of the priority value, adjusting the actual inlet temperature of said first compressor to the determined desired inlet temperature, and adjusting the actual speed for each compressor to the determined desired speed for each compressor.

2. The method according to claim 1, wherein the speed index for each compressor corresponds to the ratio of the difference between the maximum speed and the actual speed of each compressor, and the maximum speed.

3. The method according to claim 1, wherein the priority value influences the control in such a manner that if the smallest speed index of all compressors is smaller than the proportional value, the actual inlet temperature will be lowered, until the proportional value is smaller than the smallest speed index.

4. The method according to claim 1, wherein the actual speed of each compressor is determined from a reduced actual speed, and the desired speed of each compressor is determined from a reduced desired speed, wherein the reduced actual speed is determined from the actual speed and an actual temperature at the entry of the respective compressor, and wherein the reduced desired speed is determined from the desired speed at the entry of each compressor.

5. The method according to claim 1, further comprising determining an integral value from the priority value, wherein the integral value is used to determine a reduced set speed of the respective compressor.

6. The method according to claim 1, further comprising determining an actual total pressure ratio, wherein the actual total pressure ratio corresponds to the quotient of an actual outlet pressure corresponding to the pressure at an outlet of the farthest upstream compressor, and the actual inlet pressure of the first compressor.

7. The method according to claim 6, wherein a capacity factor is determined from the actual total pressure ratio and a proportional-integral value of the priority value and an integral value is determined, wherein a reduced desired speed for each compressor is determined as a functional value of control function attributed to the respective compressor, which assigns a reduced desired speed to each value pair, from capacity factor and model total pressure ratio, which is determined by or equal to the actual total pressure ratio.

8. The method according to claim 3, wherein, if the smallest speed index of all compressors is smaller than the proportional value, the actual inlet temperature is lowered by gradually lowering the determined desired inlet temperature.

9. The method according to claim 3, wherein the actual speeds of the compressors are not increased as long as the smallest speed index is smaller than the proportional value.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The following illustration descriptions detail preferred variants and examples, as well as other features of the method according to the invention:

(2) The drawing FIGURE is a schematic illustration of the method according to the invention.

(3) The drawing figure is a schematic illustration of a process diagram, which can be used for implementing the method according to the invention. Four compressors V.sub.1, V.sub.2, V.sub.3, V.sub.4 are arranged in a series, and each features an inlet pressure p.sub.actual, p.sub.1, p.sub.2, p.sub.3 at its suction side and a temperature T.sub.actual, T.sub.1, T.sub.2, T.sub.3 at its entry point. Upstream of the first compressor V.sub.1 of the series, there is an inlet for cold fluid at a temperature T.sub.coldbox (for example 200K, 100K, 50K, 20K and/or 4K), which can be added to the fluid requiring cooling in particular via a valve. For each compressor V.sub.1, V.sub.2, V.sub.3, V.sub.4, temperature T.sub.actual, T.sub.1, T.sub.2, T.sub.2, T.sub.3 is determined at entry point. For the first compressor V.sub.1 this is the actual inlet temperature T.sub.actual. Furthermore, the actual pressure p.sub.actual, p.sub.1, p.sub.2, p.sub.3 is also determined at the input of the respective compressor V.sub.1, V.sub.2, V.sub.3, V.sub.4. An actual total pressure ratio .sub.actual is calculated from the actual inlet pressure p.sub.actual and the actual outlet pressure p.sub.4. This serves to determine the reduced speeds n.sub.1desired, red, n.sub.2desired, red, n.sub.3desired, red, n.sub.4desired, red of compressors V.sub.1, V.sub.2, V.sub.3, V.sub.4:

(4) ${}_{\mathrm{actual}}-\frac{p_4}{p_{\mathrm{actual}}}$

(5) From the actual and desired inlet pressures p.sub.actual, p.sub.desired as well as the actual total pressure .sub.actual, it is possible to determine a capacity factor X that is equal to all compressors V.sub.1, V.sub.2, V.sub.3, V.sub.4. This capacity factor X serves to determine for each compressor V.sub.1, V.sub.2, V.sub.3, V.sub.4 the respective reduced desired speeds n.sub.1desired, red, n.sub.2desired, red, n.sub.3desired, red, n.sub.4desired, red via a control function F attributed to each respective compressor V.sub.1, V.sub.2, V.sub.3, V.sub.4 (pre-calculated for each compressor in the form of e.g. a table or a polynome) so that the compressors V.sub.1, V.sub.2, V.sub.3, V.sub.4 of the series work in a most economical manner.

(6) The capacity factor X in particular is of such nature that it can accept values between 0 (X.sub.pump=0 pumping regime) and 1 (X.sub.block=1, blocking regime). Both the pumping and the blocking regimes are operating conditions of the compressor, which should be avoided. The pumping regime corresponds to the operating states, in which the compressor satisfies the so-called surge condition whereas, on the other hand, the blocking regime corresponds to operating conditions that meet the so-called choke condition. In order for the compressors not to enter these regimes, the capacity factor X gets limited to values between a minimum value X.sub.min=X.sub.pump+0.05 and a maximum value X.sub.max=X.sub.block0.1.

(7) Likewise, for the integral value int.sub.t=n+1, an upper and a lower limit value int.sub.max and/or int.sub.min of integral value int are derived via X.sub.max and/or X.sub.min and from the natural logarithm of the actual total pressure ratio ln(.sub.actual):
int.sub.min=X.sub.min+ln(.sub.actual)
int.sub.min=X.sub.min+ln(.sub.actual).

(8) Since the measured actual total pressure ratio .sub.actual continues to increase during transient mode (pump-down) (the actual inlet pressure p.sub.actual continues to decrease), the limits of the integral value also increase. In the opposite case (pump-up), i.e. if the desired inlet pressure p.sub.desired is smaller than the actual inlet pressure p.sub.actual, those limit values continue to decrease.

(9) If the integral value int.sub.t=n+1 is greater and/or smaller than the upper and/or lower limit value int.sub.max, int.sub.min, it will be limited to the respective limit value.

(10) Priority value PW and integral value int.sub.t=n+1 are added together in order to generate a proportional-integral PI value:
PI=PW+int.sub.n+1

(11) If all compressors V.sub.1, V.sub.2, V.sub.3, V.sub.4 run in series at their specification points, the compressor series reaches its design or operating at a design total pressure ratio .sub.design.

(12) If the proportional-integral value PI is smaller than the sum of the maximum value of the capacity factor X.sub.max and than the natural logarithm of the design total pressure ratio value .sub.design, the capacity factor X is determined from the difference of the proportional-integral value PI and the natural logarithm of the actual total pressure ratio .sub.actual. Otherwise, the proportional-integral PI value is limited to the sum of the natural logarithm of the design total pressure ratio .sub.Design and the maximum value of the capacity factor X.sub.max in particular when determining capacity factor X. The following thus applies:
X=PIln(.sub.actual).sub.if PI<ln(.sub.Design)+X.sub.block
X=ln(.sub.Design)+X.sub.blockln(.sub.actual),otherwise
based on the capacity factor X determined in such manner, the process according to the invention now chooses how a model total pressure ratio .sub.model is determined, which is then handed to the control function F for determining the reduced desired speeds n.sub.1desired, red, n.sub.2desired, red, n.sub.3desired, red, n.sub.4desired, red. The model total pressure ratio .sub.model is equal to the actual total pressure ratio .sub.actual, provided the determined capacity factor X is situated between the minimum and maximum values X.sub.min, X.sub.max is. Provided the capacity factor X is outside this value range, then the model total pressure ratio .sub.Model is altered is altered via a saturation function SF.

(13) Subsequently, the capacity factor X is limited to its minimum and/or maximum values X.sub.min, X.sub.max is restricted. In particular, in conjunction with the model total pressure ratio .sub.model, it is redirected to control function F, which uses these arguments as foundation to determine the reduced desired speeds n.sub.1 desired red, n.sub.2 desired, red, n.sub.3 desired, red, n.sub.4 desired, red for the respective compressors V.sub.1, V.sub.2, V.sub.3, V.sub.4.

(14) The saturation function SF can be given for values of the capacity factor X, which are not situated between the minimum and the maximum values X.sub.min, X.sub.max, for example via
SF=exp(0,5*(XX.sub.max)).sub.for X>X.sub.max
and/or
SF=exp(0,5*(XX.sub.min)).sub.forX<X.sub.min
This means:
.sub.Model=.sub.actual.Math.SF ln(.sub.Model)=ln(.sub.actual)+0,5.Math.(XX.sub.min/max).

(15) This modification of the model total pressure ratio .sub.model ensures that in operating states in which the capacity factor X is at saturation, the control continues to nevertheless have an impact on compressors V.sub.1, V.sub.2, V.sub.3, V.sub.4, since then, the model total pressure ratio .sub.model is changed instead of the capacity factor X, allowing control function F to request reduced desired speeds n.sub.1desired, red, n.sub.2desired, red, n.sub.3desired red, n.sub.4desired leading out of these operating states.

(16) The reduced desired speeds n.sub.1desired, red, n.sub.2desired, red, n.sub.3desired red, n.sub.4desired can be deposited for each compressor V.sub.1, V.sub.2, V.sub.3, V.sub.4, especially in the form of a table (look-up table). This table can be created in particular by model calculations using Euler's turbomachinery equations. In accordance with capacity factor X and the model total pressure ratio .sub.Model, a software for reading the reduced desired speeds n.sub.1desired, red, n.sub.2desired, red, n.sub.3desired, red, n.sub.4desired from the table can be used. This table then corresponds in particular the control function F and comprises, at least for a number of capacity factors X (for example, X=0, 0.25, 0.5, 0.75 and 1), and model total pressure ratios .sub.model the respective reduced speeds n.sub.1desired, red, n.sub.2desired, red, n.sub.3desired red, n.sub.4desired for the respective compressor V.sub.1, V.sub.2, V.sub.3, V.sub.4. Values of the capacity factor X not listed in the table, are determined by interpolation. Furthermore, the capacity factor X as a function of the model total pressure ratio .sub.Model and reduced speeds n.sub.1desired, red, n.sub.2desired, red, n.sub.3desired red, n.sub.4desired n.sub.red is chosen so that the actual inlet pressure p.sub.actual aligns with the desired inlet pressure p.sub.desired via the control function F.

(17) In order to ensure a system pump-down in parallel with the cool-down, i.e. reducing the pressure to the suction side of compressors V.sub.1, V.sub.2, V.sub.3, V.sub.4 during the cooling phase, it must be decided whether the actual inlet temperature T.sub.actual must be lowered at the entry of the first compressor V.sub.1 in order to avoid excessively high speeds in the compressors V.sub.1, V.sub.2, V.sub.3, V.sub.4 or whether the operation can be ensured without additional cooling at the entry of the first compressor V.sub.1. For this purpose, two values are compared with each other. At first, a proportional value prop is calculated from the actual and desired inlet pressures p.sub.actual, p.sub.desired. Then, a speed index is calculated from a speed quota for each compressor calculated. And secondly, a speed index is calculated for each compressor from a speed quota, wherein the speed quota is given by

(18) $Q_i=\frac{n_i}{n_{i,\max }}$
and the speed index D.sub.i is given by

(19) $D_i=1-Q_i=1-\frac{n_i}{n_{i,\max }}$
where n.sub.i,max equals the maximum speed of the respective compressor V.sub.i. i is an index (i=1-4).

(20) Hence, if the speed index D.sub.i of a compressor V.sub.i tends towards zero, this means that compressor V.sub.i is operating near its maximum speed n.sub.i, max, and no higher speeds n.sub.i should be set by increasing the reduced desired speeds n.sub.1desired, red, n.sub.2desired, red, n.sub.3desired red, n.sub.4desired, red.

(21) From the amount of speed indices D.sub.i for each compressor V.sub.i, the smallest speed index D.sub.i will now be compared with the proportional value prop. The smaller of the two values is assigned to priority value PW, which then serves to determine further control values (such as for example the reduced desired speeds n.sub.1desired, red, n.sub.2desired, red, n.sub.3desired red, n.sub.4desired red, in particular by means of the capacity factor or the desired inlet temperature T.sub.desired). This means that if a compressor V.sub.i already operates at very high speeds n.sub.i, its speed index D.sub.i will be nearly or equal to zero. This prioritizes the system control in a manner as to adding cold fluid upstream of the inlet of the first V.sub.1 via a cooling reservoir, so that the actual inlet temperature T.sub.actual is lowered. As a result, speeds n.sub.i compressors V.sub.i, decrease, so that the speed index D.sub.i of this compressor V.sub.i increases againand namely, in particular, until the proportional value prop will be lower. This ensures an an economical operation of the compressor series, especially during the cool-down and pump-down phases.

(22) From the priority value PW, a temperature control unit TE determines the desired inlet temperature T.sub.desired. Throughout, the calculation is of a qualitative nature as to ensure that in case of a low priority value PW, the desired inlet temperature T gets gradually reduced. For example, the desired inlet temperature T.sub.actual can be set at 90% of the most recently measured actual inlet temperature T.sub.actual. The downgrade to this value can for example be realized via a ramp function. If during the downgrading of the desired inlet temperature T.sub.desired, the speed indices still enjoy priority status, the desired inlet temperature the T.sub.actual will be newly reduced to 90% of the last measured actual inlet temperature T.sub.actual. For each downgrade of the desired inlet temperature T.sub.actual to 90% of the measured actual inlet temperature T.sub.actual, it will be verified whether the determined desired inlet temperature T.sub.desired is greater than a specified temperature at the inlet of the compressor series. Provided the specified temperature is 4K, and the temperature desired value is 3.8 K, then the value will be limited to 4K.

(23) Via a cooling reservoir control box C, the respective amount of cold fluid will be impinged on the warm fluid upstream of the entry of the first compressor V.sub.1 so that by mixing the two differently warm fluids, the fluid has a mixture temperature that is lower than the previously measured actual inlet temperature T.sub.actual. At a higher priority value PW, the at the inlet of the first compressor V.sub.1 will be impinged on with no or only a small amount of cold fluid, since compressors V.sub.1, V.sub.2, V.sub.3, V.sub.4 of the series already run at non-excessive speeds n.sub.1.

(24) In a variant of the invention, an integrator, which is in particular part of a PI (proportional-integral) controller, and which carries out a temporal integration of the priority value PW, can also impact the calculation of the desired inlet temperature T.sub.desiredfor example in a manner as to reach a certain steepness of a temperature ramp for T.sub.desired.

(25) It is important throughout the entire control that reduced values for controlling the system and, in particular, compressors V.sub.1, V.sub.2, V.sub.3, V.sub.4 be used. The reduced speed n.sub.i,red of a compressor V.sub.1 can thus for example be calculated via the following formula.

(26) $n_i=n_{i,\mathrm{red}}.Math.n_{i,\mathrm{Design}}.Math.\sqrt{\frac{T_{i-1}}{T_{i,\mathrm{Design}}}}$
wherein n.sub.i is the speed of the compressor (desired or actual speed), n.sub.i, red the reduced speed (desired or actual speed) of the compressor V.sub.i, n.sub.i, design the specified or design speed of the compressor V.sub.i. T.sub.i-1 the temperature at the inlet of the compressor V.sub.i, and T.sub.i, design the specified or design temperature of the compressor V.sub.i. Wherein T.sub.0(.sub.i=1) equals the actual inlet temperature T.sub.actual of the first compressor V.sub.1. In a parallel manner, the following applies for reduced mass flow {dot over (m)}.sub.red:

(27) ${\stackrel{.}{m}}_{\mathrm{red}}=\frac{{\stackrel{.}{m}}_{\mathrm{actual}}}{{\stackrel{.}{m}}_{\mathrm{Design}}}.Math.\frac{p_{\mathrm{Design}}}{p_{\mathrm{actual}}}.Math.\sqrt{\frac{T_{\mathrm{actual}}}{T_{\mathrm{Design}}}}$
wherein {dot over (m)}.sub.red represents the reduced mass flow through the compressor, m.sub.actual the current mass flow, {dot over (m)}.sub.Design the mass flow designating the one specified for the respective compressor, p.sub.Design the specified pressure at the respective compressor, T.sub.Design is specified temperature and p.sub.actual the actual inlet pressure at the respective compressor.

REFERENCE SIGN LIST

(28) TABLE-US-00001 PW Priority value prop Proportional value int Integral Value p.sub.ist Actual inlet pressure at first compressor p.sub.desired Desired inlet pressure at first compressor TE Temperature control unit C Cooling reservoir control box F Control function X Capacity factor D.sub.i Speed index of I compressor (i = 1-4) n.sub.i Actual speed of i compressor (i = 1-4) n.sub.i,max Maximum speed of I compressor (i = 1-4) V.sub.i I compressor of series (i = 1-4) p.sub.i Actual pressure at outlet of i compressor, and/or entry of (i + 1) compressor (i = 1-4) n.sub.i,desired Desired speed of i compressor (i = 1-4) n.sub.i,desired,red Reduced desired speed of i compressor (i = 1-4) n.sub.i,Design Specified and/or designed speed of i compressor (i = 1-4) T.sub.ist Actual inlet temperature (at first compressor) T.sub.desired Desired inlet temperature (at first compressor) T.sub.i Actual temperature at entry of (i + 1) compressor, at outlet of i compressor (i = 1-4) T.sub.i,Design Specified and/or designed temperature of i compressor (i = 1-4) T.sub.coldbox Temperature of cold fluid SF Saturation function .sub.Model Model total pressure ratio .sub.actual Actual total pressure ratio .sub.Design Design total pressure ratio X Capacity factor X.sub.min Minimum value of capacity factor X.sub.max Maximum value of capacity factor PI Proportional-Integral value