DE102017125125B3 - Method for carrying out a pressure test of a liquefied gas tank - Google Patents

Abstract

Method for performing a pressure test of a liquefied gas tank, with the steps
(a) removing liquid LPG (18) from the LPG tank (12),
(b) heating the liquid liquid gas (18) to evaporate and form gas (24), and
(c) returning the gas (24) back into the liquefied gas tank (12),
(d) carrying out steps (a) to (c) at least until a predetermined test internal pressure (p _test ) is reached in the liquefied gas _tank (12),
(e) Performing a compressive strength test after reaching the test internal pressure (p _test ).

Description

The invention relates to a method for carrying out a pressure test of a liquefied gas tank. It relates to a liquefied gas plant having (a) a liquid gas tank containing liquefied gas, (b) a pump for withdrawing liquid liquefied gas from the liquefied gas tank, (c) an evaporator for heating the liquid liquefied gas so as to vaporize and form a gas (d) a return line for returning the gas back to the liquefied gas tank, (e) a pressure gauge for measuring an internal pressure in the liquefied gas tank, and (f) a control unit.

Liquefied gas is understood as meaning a mixture of propane, butane and optionally propene, butene, isobutane and / or isobutene. It is used, for example, for heating residential buildings or in industrial plants and stored in liquefied gas tanks. In small, mobile liquefied gas containers, such as camping cookers or burners for craft supplies, gaseous liquefied gas is removed. By removing gas from the liquefied gas container, part of the liquid liquefied gas evaporates while absorbing heat of vaporization. This reduces the temperature in the LPG tank. The greater the temperature difference of the liquefied gas container to the environment, the greater the heat flow from the environment into the liquefied gas container. If a large amount of liquid gas is removed in a short time, the temperature may drop below the boiling point of the liquefied gas. In this case, no gaseous liquefied gas can be removed from the liquefied gas container.

From immobile liquid gas tanks to which the invention relates, which are larger than the mobile liquefied gas containers and in particular contain more than 500 liters of liquid gas, liquid liquefied gas is removed, evaporated by means of auxiliary energy and fed to the use. This prevents the temperature in the LPG tank from dropping and ensures that LPG can always be withdrawn since it can not cause the above-described freezing. However, this approach has the disadvantage that for the evaporation of the liquefied gas auxiliary energy must be expended. If the evaporator fails, only as much liquid gas can be removed as the natural evaporation performance of the container allows, which is disadvantageous.

A disadvantage of known systems is also that mandatory pressure tests can be performed only with great effort. For the pressure test, for example, nitrogen is pressed into the liquid gas container in order to increase the internal pressure to at least the necessary internal test pressure or the process internal pressure. The disadvantage of this is that this can change the calorific value of the liquefied gas.

The invention is based on the object to reduce disadvantages in the prior art.

The invention solves the problem by the features of claim 1. Advantageous embodiments thereof are the subject of the dependent claims. The invention solves the problem in particular

by a method for carrying out a pressure test of a liquefied gas tank with the steps (a) to (c), wherein these steps are carried out at least until a predetermined test internal pressure or process internal pressure is achieved in the liquefied gas tank. This test internal pressure can be, for example, in an interval of 11 bar to 13 bar.

For this purpose, a liquid gas system is presented, in which the control or regulating unit is designed to automatically control or regulate a volume flow of the pump and a heating power of the evaporator, so that the internal pressure is increased to a predetermined test internal pressure or process internal pressure and / or increases ,

An advantage of the invention is that the need for additional heating energy can be significantly reduced. Thus, as described above, the removal of gaseous liquefied gas from the liquefied gas tank causes its temperature to drop. Unlike in the past, however, this is not considered a problem because the heat flow from the environment then increases in the liquefied gas tank. The amount of heat so flowed into the liquid gas tank no longer needs to be used for heating the liquid liquefied gas. It is therefore sufficient to heat the liquid liquid gas (and preferably only then) when the temperature in the liquefied gas tank is so low that it is no longer possible to remove a sufficient amount of gaseous liquid gas.

Preferably, a volume flow of liquid LPG when removing the liquid liquefied gas from the liquefied gas tank and a heating power during heating of the liquid liquefied gas are so selected and optionally adjusted that when the internal test pressure or process internal pressure reaches an average temperature in the liquefied gas tank at least 5 Kelvin, in particular at least 8 Kelvin, is below the temperature corresponding to the vapor pressure curve of the liquefied gas to the test internal pressure or process internal pressure. If gaseous liquefied gas is removed from the liquefied gas tank, its temperature decreases, as described above. Will be liquid LPG removed, evaporated and returned, so the pressure in the LPG tank increases. It would therefore be expected that this also increases the temperature in the liquefied gas tank.

However, if a sufficiently large amount of gaseous liquefied petroleum gas is fed into the tank, the pressure rises to a value higher than would be expected from the average temperature. This has the consequence, for example, that after the end of the supply of gas in the liquefied gas tank, the pressure drops relatively quickly again, even if no gaseous liquefied gas is removed. The reason for this is that the liquid liquefied gas heats up much more strongly when the gas is returned at its upper side than further away from the liquid gas surface. There is therefore a temperature gradient within the liquid liquefied gas. This temperature gradient compensates again after the end of the process. A corresponding example is explained in more detail below in the description of the figures.

The average temperature can therefore be determined, for example, by the fact that the return of the gas is prevented and no gaseous liquefied gas is removed more from the liquefied gas tank. After a waiting time of, for example, 180 minutes, an internal pressure then arises in the liquefied gas tank. From the vapor pressure curve of the liquefied gas, the average temperature can be calculated from this internal pressure. In particular, the average temperature is understood to be the temperature which is determined in this way.

The size of the heating capacity to be selected and the size of the volume of liquid LPG to be used in order for the heating capacity to evaporate completely are determined in preliminary tests.

According to a preferred embodiment, the volume flow and the heating power are selected so that the internal pressure changes at an internal pressure change rate of at least 0.5, in particular at least 1 bar, per 10 minutes.

Alternatively or additionally, the volume flow when removing the liquid liquefied gas and / or the heating power during heating of the liquid liquefied gas is selected such that upon reaching the test internal pressure or the process internal pressure, the liquid gas in the liquefied gas tank has an equilibrium internal pressure which is at least 1 bar is below the test internal pressure. Under the equilibrium internal pressure of that internal pressure is understood, which sets after a waiting time of for example 30 minutes after reaching the test internal pressure without further return of gas in the liquefied gas tank and without removal of gaseous liquefied gas. As described above, the corresponding heating power and the corresponding volume flow are determined in a preliminary test.

Preferably, the liquefied gas is taken up, in particular by means of a dip tube, from the liquefied gas tank. It is known to subtract liquid gas from the bottom of the liquefied gas tank to then heat it up and supply it to the consumption. The evaporation unit is also arranged below the liquid gas tank, so that the liquefied gas can be withdrawn by gravity.

For efficient operation of the evaporator, it is advantageous if it is charged with LPG, which is at a pressure which is higher than the pressure downstream of the evaporator. The latter pressure usually corresponds essentially to the internal pressure of the liquid gas tank. The LPG system therefore has a pump for increasing the pressure of the liquid liquid gas and for pumping the liquid liquid gas to the evaporator. Since the pump is present anyway, it is advantageous to form this as a suction pump. It is then possible to remove the liquid gas by means of a dip tube upwards from the liquid gas tank.

This has the advantage that the tank can be easily formed. Preferably, the evaporator is arranged at a height that is greater than the lowest point of the liquid gas tank. The removal of the liquid liquid gas upwards is particularly advantageous in earth-covered liquid gas tanks, as a complex substructure is eliminated.

Preferably, the liquid liquid gas is removed from the liquefied gas tank by means of a positive displacement pump which is driven by an explosion-proof electric motor. So far, these pumps are driven by compressed air because they must be operated in a potentially explosive atmosphere. These are difficult to fix. In particular, in order to be able to regulate and / or control the volume flow of liquid natural gas in a particularly precise manner, it is advantageous to use a positive-displacement pump, in particular a power-controlled one.

Preferably, the liquid liquefied gas is taken from an underground covered liquefied gas tank. The surrounding soil can deliver heat to the liquefied gas tank so that the minimization of heating power can be particularly clear.

Preferably, the liquefied gas tank has a volume of at least 500 liters, in particular at least 3000 liters, more preferably at least 5000 liters. The larger the volume of the liquid gas tank, the more difficult it becomes to increase the pressure in the liquid gas tank, and the more advantageous the solution according to the invention is.

Preferably, after the test internal pressure has been reached, a pressure resistance test, in particular a sound emission test, is carried out. The sound emission test records the sound emitted by the LPG tank material. The acoustic emission test is especially after DIN EN ISO 9712: 2011 carried out. In this case, it is usually a replacement test for conventional testing of pressure equipment.

Preferably, the heating power is controlled or controlled so that an average temperature in the liquefied gas tank does not rise. When removing LPG from the liquefied gas tank, as described above, the temperature drops. If the heating power is controlled or controlled so that the average temperature in the liquefied gas tank does not increase, it is avoided to supply unnecessary heating energy. In particular, it is advantageous if the heating power is only increased when a predetermined minimum temperature is reached.

Preferably, the method comprises the automatically performed steps of (a) continuously monitoring an internal temperature and / or internal pressure in the liquefied gas tank (b) heating the liquid liquefied gas so that it vaporizes and gasises, only if the internal temperature falls below a minimum indoor temperature and / or the internal pressure falls below an internal pressure minimum value. As described above, this ensures that only as much heating energy is used as is necessary.

Preferably, the pump is driven by an explosion-proof electric motor positive displacement pump. The pump preferably comprises a frequency converter, so that a volume flow which the pump delivers, is controllable. Preferably, the evaporator is directly electrically heated and / or indirectly heated, in particular by means of hot water. For this purpose, the liquefied gas plant preferably comprises a hot water tank.

It is particularly advantageous if the liquefied gas plant also has a solar thermal system for heating the water in the hot water tank. Alternatively or additionally, the hot water can be heated by means of gas.

The LPG system preferably comprises a safety shutdown device, which is designed to switch off the pump and / or the evaporator, if a predetermined target internal pressure is reached or exceeded.

The liquefied gas plant also has a removal device for removing gaseous liquefied gas from the liquefied gas tank. This ensures that gaseous liquefied gas is always available, even if the evaporator is not or is not yet ready for use.

• 1 ein Schaltbild einer erfindungsgemäßen Flüssiggasanlage zum Durchführen eines erfindungsgemäßen Verfahrens und
• 2 eine Dampfdruckkurve von Flüssiggas.
• 3 zeigt den zeitlichen Verlauf des Drucks in einem Flüssiggastank beim Durchführen eines erfindungsgemäßen Verfahrens.

In the following the invention will be explained in more detail with reference to the accompanying drawings. It shows

• 1 a circuit diagram of a liquid gas installation according to the invention for carrying out a method according to the invention and
• 2 a vapor pressure curve of LPG.
• 3 shows the time course of the pressure in a liquefied gas tank in carrying out a method according to the invention.

1 shows a liquid gas system according to the invention 10 with a liquefied gas tank 12 that is below a surface of the earth 14 located. The liquefied gas plant 10 includes a pump 16 in the form of a positive displacement pump, which is designed as a suction pump and is driven by an explosion-proof electric motor. The pump sucks liquid LPG 18 by means of a dip tube 20 on.

In the gas flow direction R is in front of the pump 16 preferably a safety fitting 22 arranged, by means of which the supply of liquefied gas 18 to the pump 16 can be prevented. The pump 16 promotes liquid LPG 18 to an evaporator 23 in which the liquid LPG 18 evaporated and a gas 24 arises. The gas 24 is by means of a return line 26 into the liquefied gas tank 12 returned, preferably from above.

By means of a sampling line 28 that is a valve 30 and a pressure regulator 32 includes, gas can 24 to a schematically drawn consumer 34 be directed.

The liquefied gas plant 10 includes a pressure gauge 36 and optionally a thermometer 38 , The pressure gauge 36 measures the pressure p preferably as in 1 shown at the top of the LPG tank 12 where the gas is 24 located. The pressure gauge 36 is like the optional thermometer 38 with a control unit 40 connected. The control unit 40 is for controlling the pump 16 connected to it, so that a volumetric flow V on liquid LPG 18 from the pump 16 is promoted, controllable or regulated. Is the control unit for regulating the flow rate V trained, so includes the liquefied gas system 10 a not shown Flow meter in gas flow direction R behind the pump 16 and in front of the evaporator 23 ,

The evaporator 23 includes a heater 42 and preferably a heat storage 44 , For example in the form of a hot water tank. The heat from the heat storage 44 or the heater 42 is for example by means of a heat exchanger to the liquid LPG 18 transfer.

Schematically plotted that the heater 42 an electrical resistance element 46 can have for heating with electric current. The heat storage 44 may alternatively or additionally with a schematically drawn solar thermal system 48 be connected to produce warm water. Of course, it is also possible that as an alternative or in addition to the solar thermal system 48 a photovoltaic system is provided, by means of the electric current for operating the resistance element 46 is available.

For performing a method according to the invention is liquid LPG 18 from the liquefied gas tank 12 taken by means of the evaporator 23 evaporated and then into the liquid tank 12 returned. This is done until the internal pressure p reaches a predetermined test pressure p _test . Thereafter, a pressure test of the LPG tank 12 performed, for example, a sound emission measurement.

For increasing the internal pressure P from an actual pressure p _is on the testing internal pressure p _test, a volume flow V is taken, for example, is greater than 0.01 parts per thousand of the volume V of the liquid gas tanks 12 per second.

Thus, no inert gas is used to increase the pressure. It is possible, and represents a preferred embodiment, that a gas temperature T _{24 is} at most 5 Kelvin above the boiling point of the liquefied gas. The control unit 40 controls the evaporator 23 so that its heating capacity is sufficient to control the volume flow V produced by the pump 16 is encouraged to completely evaporate.

As a result of the increase in volume during evaporation, the internal pressure p in the liquefied gas tank rises 12 on. An average temperature T _ave in the liquefied gas tank 12 however, increases only slightly.

2 shows vapor pressure curves of propane, butane and a mixture of 50% propane and 50% butane, which is the composition of the liquefied gas in the present example. At an average temperature T _ave of 20 degrees arises in the liquid tank 12 a pressure of P = 4.2 bar. If the method according to the invention for increasing the pressure is carried out, the internal pressure p rises to, for example, 11 bar, which represents a test internal pressure p _test . This test internal pressure p _test corresponds to a temperature T _ptest of 46 ° C. The average temperature T _ave in the liquid _tank 12 However, it is much lower. If the recirculation of gas is therefore terminated, the internal pressure p rapidly drops back to a value which is just above the outlet pressure of the example 4.2 bar.

For carrying out an alternative method according to the invention, liquid LPG is used 18 from the liquefied gas tank 12 taken by means of the evaporator 23 evaporated and then into the liquid tank 12 returned. This is carried out until the internal pressure p reaches a predetermined process internal pressure p _pr . This process internal pressure p _pr represents the internal pressure, which is particularly favorable for the removal of the gaseous liquefied gas.

3 shows the time course of the internal pressure p in carrying out a method according to the invention. The initial internal pressure is 5.5 bar. By returning gaseous liquid gas, here: propane, the pressure is increased up to the test internal pressure p _test and the pressure test is carried out. Thereafter, the return is terminated and the internal pressure p slowly decreases again. It can be seen that the entire pressure test lasts less than 1.5 hours, which is less than a third of the time previously required.

LIST OF REFERENCE NUMBERS

10

LNG plant

12

LPG tank

14

earth's surface

16

pump

18

liquid LPG

20

dip tube

22

safety device

23

Evaporator

24

Gas (gaseous LPG)

26

return

28

withdrawal line

30

Valve

32

pressure regulator

34

consumer

36

pressure gauge

38

thermometer

40

Control unit

42

heater

44

heat storage

46

resistive element

48

solar thermal plant

R

Gas flow direction

V

flow

p _test

Testing internal pressure

p _pr

Process pressure

p

internal pressure

P _is

Actual pressure

V

Volume of LPG tank

T ₂₄

gas temperature

T _ave

Average temperature

Claims (9)

Method for carrying out a pressure test of a liquefied gas tank, comprising the steps of (a) removing liquid liquefied gas (18) from the liquefied gas tank (12), (b) heating the liquid liquefied gas (18) so that it vaporizes and forms gas (24), and (c) returning the gas (24) back into the liquefied gas tank (12), (d) performing steps (a) through (c) at least until a predetermined _check internal pressure (p _test ) is reached in the liquefied gas _tank (12) (e) Performing a compressive strength test after reaching the test internal pressure (p _test ). Method according to Claim 1 , characterized in that the pressure strength test is a sound emission test according to DIN EN ISO 9712: 2011. Method according to one of the preceding claims, characterized in that a volume flow (V) during removal of the liquid liquid gas (18) and a heating power during heating of the liquid liquefied gas (18) are selected so that upon reaching the test internal pressure (p _test ) an average temperature in the liquefied gas tank (12) is at least 5 Kelvin, in particular at least 8 Kelvin, below the temperature which, according to the vapor pressure curve of the liquefied gas (18), corresponds to the test internal pressure (p _test ). Method according to one of the preceding claims, characterized in that a volume flow (V) during removal of the liquid liquid gas (18) and a heating power during heating of the liquid liquefied gas (18) are selected so that upon reaching the test internal pressure (p _test ) the liquid gas (18) in the liquid gas tank (12) has an equilibrium internal pressure which is at least 1 bar below the test internal pressure (p _test ) or the process internal pressure (p _pr ), the equilibrium internal pressure being that internal pressure (p) is, which sets after a waiting time of 180 minutes after reaching the test pressure without further return of the gas (24) to the liquid gas tank (12). Method according to one of the preceding claims, characterized in that the liquid gas is removed from the liquid gas tank (12) upwards, in particular by means of a dip tube (20). Method according to one of the preceding claims, characterized in that the liquid liquid gas by means of a positive displacement pump, which is driven by an explosion-proof electric motor takes place. Method according to one of the preceding claims, characterized in that - the liquid liquefied gas is taken from a buried liquefied gas tank (12) and / or - a liquefied gas tank (12) having a volume of at least 500 liters. Method according to one of the preceding claims, characterized in that the heating power is controlled or controlled so that an average temperature (Tave) in the liquefied gas tank (12) does not rise. Method according to one of the preceding claims, characterized by the steps: (a) measuring an internal temperature and / or the internal pressure (p) in the liquefied gas tank (12) and (b) heating the liquid liquefied gas (18) so that it evaporates and gas ( 24), only if the internal temperature falls below an internal temperature minimum value and / or the internal pressure (p) falls below an internal pressure minimum value.

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