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Fault diagnosis and analysis of differential pressure transmitter in application

一、  差压变送器的工作原理:

1、 Working principle of differential pressure transmitter:


The differential pressure from the two-sided pressure pipes directly acts on the two-sided isolation diaphragm of the transmitter sensor, and is transmitted to the measuring element through the sealing liquid in the diaphragm. The measuring element converts the measured differential pressure signal into corresponding electrical signal and transmits it to the converter. After amplification and other processing, it becomes the standard electrical signal output.


2、 Several application measurement methods of differential pressure transmitter:

1.  与节流元件相结合,利用节流元件的前后产生的差压值测量液体流量。

1. Combined with the throttling element, the liquid flow is measured by the differential pressure between the front and rear of the throttling element.

2.  利用液体自身重力产生的压力差,测量液体的高度。

2. Measure the height of the liquid by using the pressure difference generated by the gravity of the liquid itself.

3.  直接测量不同管道、罐体液体的压力差值。

3. Directly measure the pressure difference of liquid in different pipes and tanks.

三、  应用中的故障判断及分析

3、 Fault diagnosis and analysis in application


In the measurement process of transmitter, there are often some faults. It is very important to judge, analyze and deal with the faults in time. According to our daily maintenance experience, we summarized some judgment analysis methods and analysis processes.

1.  调查法:回顾故障发生前的打火、冒烟、异味、供电变化、雷击、潮湿、   误操作、误维修。

1. Investigation method: review the ignition, smoke, odor, power supply change, lightning, humidity, misoperation and maintenance before the failure.

2.    直观法:观察回路的外部损伤、导压管的泄漏,回路的过热,供电开关状态等。

2. Intuitive method: observe the external damage of the circuit, leakage of the pressure pipe, overheating of the circuit, power supply switch state, etc.

3.  检测法:

3. Detection method:

1) 断路检测:将怀疑有故障的部分与其它部分分开来,查看故障是否消失,如果消失,则确定故障所在,否则可进下步查找,如:智能差压变送器不能正常Hart远程通讯,可将电源从表体上断开,用现场另加电源的方法为变送器通电进行通讯,以查看是否电缆是否叠加约2kHz的电磁信号而干扰通讯。

1) Open circuit detection: separate the suspected fault part from other parts to check whether the fault disappears. If it disappears, determine the fault location. Otherwise, the next step can be found. For example, if the intelligent differential pressure transmitter cannot communicate normally with HART remote communication, the power supply can be disconnected from the body, and the transmitter can be powered on for communication by adding additional power on site to check whether the cable is overlapped About 2KHz electromagnetic signal interferes with communication.

2)  短路检测:在保证安全的情况下,将相关部分回路直接短接,如:差变送器输出值偏小,可将导压管断开,从一次取压阀外直接将差压信号直接引到差压变送器双侧,观察变送器输出,以判断导压管路的堵、漏的连通性。

2) Short circuit detection: under the condition of ensuring safety, the relevant part of the circuit is directly short circuited. For example, if the output value of the differential transmitter is too small, the pressure conducting pipe can be disconnected, and the differential pressure signal can be directly led to both sides of the differential pressure transmitter from the outside of the primary pressure taking valve, and the output of the transmitter shall be observed to judge the plugging and leakage connectivity of the pressure pipeline.

3) 替换检测:将怀疑有故障的部分更换,判断故障部位。如:怀疑变送器电路板发生故障,可临时更换一块,以确定原因。

3) Replacement detection: replace the part suspected of failure and judge the fault position. For example, if the transmitter circuit board is suspected to be faulty, a temporary replacement can be made to determine the cause.


4) Division detection: the measurement circuit is divided into several parts, such as: power supply, signal output, signal transmission, signal detection, according to the parts of inspection, from simple to complex, from the surface to the inside, narrow the scope, find out the fault location.


4、 Fault analysis of several typical measuring circuits


Now I only take the pressure pipe fault as an example to analyze the differential pressure transmitter measurement circuit fault.

1. 导压管堵塞:

1. The pressure pipe is blocked:


In the instrument maintenance, because the differential pressure transmitter pressure pipe discharge is not timely, or the medium is dirty, sticky and other reasons, the positive and negative pressure pipe is often blocked.


When the actual flow rate decreases from before f to after F, the static pressure in the pipeline will also decrease accordingly, and the decrease value is set as P0; at the same time, when the actual flow rate decreases to F, the p-value will also increase because of the decrease of the fluid velocity in the pipe, and the increase value is set as P0 '.

即:△P=( P+-P0 )-( P-+P0′)此时变送器输出值应减小。

That is, △ P = (P + - P0) - (P - + P0 '), and the output value of the transmitter should be reduced.

2. 正导压管泄漏:

2. Leakage of positive pressure pipe:

实际上,当泄漏量非常小的时候,由于种种原因,工艺操作或仪表维修护人员很难发现,只有当泄漏量大,所测流量与实际流量相比有较大误差时才会发现,这时即使是实际流量上升,总是△P泄漏后<<△ P泄漏前,F泄漏后<3.    平衡阀泄漏:

In fact, when the leakage is very small, due to various reasons, the process operation or instrument maintenance personnel can hardly find it. Only when the leakage is large and there is a large error between the measured flow and the actual flow, even if the actual flow rate rises, it is always after △ P leakage < < before △ P leakage and after f leakage < 3

设泄漏前压力为P1,泄漏后压力为P2, P1= P1+- P1- ,F1为平衡阀泄漏前的变送器输出值,F2为平衡阀泄漏后的变送器输出值。

Set the pressure before leakage as P1, the pressure after leakage as P2, P1 = P1 + - P1 -, F1 is the output value of transmitter before balance valve leakage, and F2 is the output value of transmitter after balance valve leakage.


We assume that the fluid flow rate in the pipeline does not change, and the leakage pressure is PS,


Then: the static pressure of the positive and negative pressure pipes after leakage is:

P2+= P1+-PS,P2-= P1-+ PS

P2+= P1+-PS,P2-= P1-+ PS

P2= P2+- P2- = P1-2 PS,

P2= P2+- P2- = P1-2 PS,

根据差压与流量的关系得出F24.    气体流量导压管积液情况下的变送器测量误差:

According to the relationship between differential pressure and flow rate, F24 is obtained


Set the pressure of positive pressure pipe pressure point as P 0 +, negative pressure pipe pressure point as P 0 -, pressure of transmitter "> positive end of differential pressure transmitter is P1 +, and pressure of negative end of transmitter" > differential pressure transmitter is P1 -.

P0= P0+- P0-

P0= P0+- P0-

P1= P1+- P1-

P1= P1+- P1-


Under normal measurement:

P0= P1

P0= P1

设正常测量状态下的流量为F,则  F=K

If the flow rate under normal measurement condition is f, then f = K

这里 K为常系数。

Here K is a constant coefficient.


If the density of liquid water is ρ, then when the height of effusion in positive pressure pipe is H +, and that in negative pressure pipe is H -:

P1+= P0++ρgh+

P1+= P0++ρgh+

P1-= P0-+ρgh-

P1-= P0-+ρgh-

P1= P1+- P1-= (P0+)+(ρ h+)-( P-+ρ h-)= P+ρ (h+-h-)

P1= P1+- P1-= (P0+)+(ρ h+)-( P-+ρ h-)= P+ρ (h+-h-)


Then the transmitter output is:



当h+>h-时  变送器实际测得的差压增大,输出流量信号增大。

When H + > H -, the differential pressure measured by the transmitter increases and the output flow signal increases.


When H + here, due to the positive pressure pipe pressure measurement method, with the increase of time, H + is gradually greater than H -, and the measured flow also increases.